Factory-bought PCs are already made to be juiced up with the highest quality video card, largest chunk of RAM, most compatible keyboard/mouse combination, and sleekest design that the major companies will stick in their machines.
BUT! If you're willing to spend fewer bucks on the computer itself and a little more on the individual, more fundamental parts of the potential game-devourer, check these sites out:
CPU/Cases:
• Excellent tips at BoxGods.com!
• You could always just buy one off of a site like Xoxide.com...
Mouse/Keyboard Combo:
• Here's a WOTLK keyset for those WoW junkies.
• Ideazon is rated 4/5 by PC Magazine.
• Razerzone.com claims to use Hyperesponse™ technology in the wiring of their keyboards, which I guess aids in its 1000Hz Ultrapolling™ / 1ms response time. Does anyone want to test this stuff out?
Speakers:
• Best computer speakers review by Rich Brown.
• Wize.com has a neat little list.

Video Cards:
• Top 15 Video Graphics Cards for Computer Gaming by Dave Spohn, About.com
• Best Video Cards for the Money by Tom Woligroski, Tom's Hardware
• Nvidia has some of the newest technology out there!
Sound Cards:
• MaxmimumPC.com lists news and reviews for sound cards daily.
• TestFreaks.com has you covered.
• ComputerGeeks.com has some of the best prices I've seen for sounds cards.
RAM:
• How to add RAM by Andy Walker, cyberwalker.com
• How to upgrade your PC's RAM by Kirk Steers, PC World.
• Hack Attack: How to install RAM by Adam Pash
Disk Space:
• Before purchasing more disk space, you may first want to know the difference between hard drive space and RAM... Here's a little guide for total noobs.
• You should also free up the disk space you already have. Try and clear up all of the useless debris floating around your hard drive such as obsolete programs and services that are inactive on a daily basis. You can do this manually or follow these simple steps toward saving your computer.
• DriveGuys.com has a nice list going on...
Fans:
• Don't exactly know how to install some more fans for your PC? A quick way to find out would be to watch some YouTube DIY'ers.
• Quiet Cooling Solutions and Fans & Fan Controllers by xPCgear.com
If you want a good site for price comparisons, and you have the time to research specific pieces of hardware, PriceWatch.com is spot-on. Newegg.com usually has some good deals, as well. (That's where I got my HP Slimline s3530f. Yummm...)