When are you thinking of building it? Haswell should arrive in about a month & bring with it a new socket (1150). That would allow for at least a couple of years of upgradability processor-wise.

Not that you really need it performance wise, Ivy Bridge won't be a bottleneck for gaming.

GPU-wise I agree with sticking under 300 & an SSD for the system drive is a must.

For maximum comfort also make sure to pick up some thumbscrews, cable ties & have a magnetic screwdriver and led light handy.

Also, look for low-rpm 120/140mm fans and cases with logical in/out cooling paths. Many enthusiast cases also include acoustic dampening pads. There's no reason for a gaming rig to be annoyingly buzzy.

You can build a perfectly capable PC that can play pretty much all games at high/ultra unless your son plays Crysis 3 for under 1500. Anything over and it's just unnecessary; PC components get outdated too fast to make such large investments.

I built mine years back around an i5 2500K and GTX650Ti and I swear it was perfect for the average gamer, and even pros (I was SC and CS pro). CSS is a CPU game and it gave me 1300FPS, which I had to cap to 999. SC was smooth as butter at full res w/ ultra settings. I played AC, D2, WoW, Dota, NFS, SC, CSS, CoD, Maplestory, and more, and I've never had any reason to complain.

Back then SSDs were super expensive, even 64Gb was over 120. Now they're damn cheap. Also, it doesn't hurt to get a fully modular gold rated PSU at 850W+ just to leave room for upgrades like SLI. Also, get a midtower that is known for excellent air flow; screw looks. Aesthetics stop mattering when your rig is overheating. It also doesn't hurt to get a water cooler like H80, it works wonders over air. Lastly don't overlook good wire organization to maximize airflow, and make sure you get extra GOOD fans that run quiet but has excellent airflow. You need to see how your tower will breathe and configure your fans appropriately; note you should always have more airflow out than in otherwise dust will gather fast.

I'm sure the other guys can help you out with the specific parts since I'm out of the scene now, but just reminding you that you don't need to spend so much on a gaming rig.

If he's going to be doing pvp/mmorpg w/ load screens, consider setting up a RAMdisk for instant loading and killing.  For memory I've been going w/ Mushkin lately for reliability. I've heard good things about Gskill.  Corsair and Crucial have been hit or miss over the decades, not sure who's the better these days.

For SSDs stay away from Sandforce controllers unless the new ones can do incompressible data.  I like my Samsung SSDs and their management software is brilliant.  I don't like controllers that cheat by running compression to post BS performance figures.

I used to avoid watercooling like the plague but have come around to using Corsair water blocks and radiators for my CPU.  They work great.  I'd recommend running a filter over the radiator for dust though for ease of cleaning.  I don't do mega monolitihic CPU heatsinks anymore as they invariably needed a fan anyway and just add stress to the motherboard.  I run my fans so they pull from the back of the radiator and push across the mobo and my GPU w/ its passive heatsink.  GPU fans are too loud and this works and looks bitchin'. 

For Mobos I'd look to Gigabyte, ASrock and Asus.  Be sure to check reviews on particular models within each make.

I'm a big Seasonic PSU guy.  Look for their gold and platinum rated lineup for efficiency and performance.  Mine is 760 watts and runs over 80% and uses a hybrid active/passive cooler.

I've gone back to Logitech peripherals, G9/G10 for mice and G13 for the left hand.  I used to like Razers but they just break these days.

I can definitely recommend G.skill RAM, I've had great results with them. Corsair used to be the Gold standard, not so much these days. As with everything, don't touch OCZ with a ten foot pole.

Sandforce should also be avoided at all costs. It was one thing when Sandforce was killing everybody else in sustained reads and writes, but those days are simply over. The Samsung 830 and 840 Pro drives just destroy Sandforce, PLUS you get some of the best reliability in the biz vs. Sandforce's extremely poor record of dodgy, bug ridden firmware. Once again OCZ completely sucks. Alternatively, the Plextor drives are said to be very good.

Disagree on tower heatsinks. You don't have to go insane with an Archon or Silver Arrow monster to get good cooling. A Megahalems or Genesis will do the job just fine, without damaging the board or requiring a massive case to actually fit. Use Noiseblocker fans and you'll get considerably less noise than most closed loop WC setups.

I can't recommend Gigabyte boards, I've had a 100% failure rate with mine. ASRock tend to mostly be cheapie designs, not really suited to a high-end gaming PC. Asus used to be super king bignuts, but their QC seems to have taken a dive in recent years (on the regular boards anyway) and dealing with Asus support can be a nightmare. Their ROG motherboards are usually very solid, but you pay for that privilege. Lately I've been very impressed with MSI. They used to be basket cases, but they've really gotten their act together in the last couple of years. I've worked with a few of their middle range GD65 boards and they've been great, at a considerably lower cost than the equivalent Asus.

Seasonic, yes, absolutely. Not always the very best voltage regulation or DC output, but they are usually at least in the ballpark vs. the best in the field, and more importantly, they are friggen reliable.

I need a very specific mouse shape to be comfor table, so I use a Roccat Kone which borrows the classic Logitech shape but doesn't have that shitty Logitech finish that always rubs off in about a month and makes the mouse look like it has eczema. The Roccat uses a kind of mouse fur type coating that's very comfortable and extremely grippy.

ASrock are just as suited for gaming as any Asus board. Hell, prior to a couple of years ago they shared a lot of the design and there is still a financial and manufacturing relationship between the two companies.

The current "upscale" mobo market is more about bling (oooh look at those bullet-shaped heatsinks and military-class caps) and piling on options (SupremeFX audio  )

Go by the features you need and by cumulative user & pro reviews of individual products, not by company-wide anecdotes. For instance, I've had two sticks of g.skill RAM fail and a raid array of OCZ sandforce SSDs work flawlessly for two years now.

As for how extreme you can go, that depends on the gaming style. For FPS games you'll want a monitor with low input lag, a mechanical keyboard and a mouse with a quick polling rate; for racing games you might want a three-display setup for a wraparound feel; for minesweeper you're going to need liquid nitrogen cooling, etc. First consider your budget.

Bonus protip: don't choose a power supply until you have an idea of the total power consumption at load, there's an efficiency curve to consider http://www.anandtech.com/show/2624/3

ASrock are just as suited for gaming as any Asus board. Hell, prior to a couple of years ago they shared a lot of the design and there is still a financial and manufacturing relationship between the two companies.

The current "upscale" mobo market is more about bling (oooh look at those bullet-shaped heatsinks and military-class caps) and piling on options (SupremeFX audio  )

Go by the features you need and by cumulative user & pro reviews of individual products, not by company-wide anecdotes. For instance, I've had two sticks of g.skill RAM fail and a raid array of OCZ sandforce SSDs work flawlessly for two years now.

Upscale mobos are also about high quality PCBs that are thicker than a sheet of paper, and phase power designs. I haven't looked too closely at ASRock recently, but what I remember in the past were cheap ass floppy boards with shitty power regulation.

You should open a museum, you may have the only working OCZ raid array in the world.