Okay... I've been researching PC building pretty heavily recently, and here's what I'd recommend.
Generally, after the $800 mark or so, you're going to run into heavy diminishing returns on your home-built PC. Keep this in mind as you're selecting your parts. If you spend $800 now and another $800 two years down the road for upgrades, you'll be spending much less than the guy who springs for a $3000 Uberbox and keeps it for 4-5 years without upgrading, with maybe slightly less performance.
With that in mind, here's what you should do.
CPU: Quad-core processors, as it currently stands, barely have any advantages over a tri-core in terms of performance. (Certain games may list a quad-core under their "recommended" specs, but in most performance tests quad-cores only barely beat out tri-cores.) In addition, AMD has Intel beat by a mile in terms of price:performance ratio. The best "bang for your buck," therefore, will be an AMD tri-core. The Athlon X3 435 (2.9 GHz triple-core) will let you run modern games for a few years at insanely good framerates, and it's under $100 on Newegg.
MOBO: I recommend the GIGABYTE GA-MA770T-UD3P. Packed with features, incredibly expandable reliable, and - again - under $100.
RAM: DDR3 is basically the new de-facto standard, and it's only slightly more expensive than DDR2. Get thee 4GB of DDR3 1333 RAM - anything with a higher speed is a waste of money.
GRAPHICS: ATI currently has the best price:performance ratio of the two major manufacturers, unless you're REALLY interested in the PhysX gimmick. A Radeon 4770 or 4850 should allow you to display modern games at insane framerates for the next few years at your monitor's resolution. I'd use something like this XFX HD-477A-YDFC Radeon HD 4770 512MB. But if you're still dead-set on NVIDIA, just know that - where graphics cards are concerned - anything over the $130-$140 mark is a terrible waste of money.
HDD: You'll want at least a 500GB one, for obvious reasons. Just grab whichever one has high ratings and a reasonable price on Newegg.
CASE: The standard "cheap-but-effective" case seems to be the COOLER MASTER Centurion 5. I really wouldn't recommend going any more expensive than that unless you absolutely need a whisper-quiet system.
PSU: Contrary to somewhat widespread belief, a good 500-550W PSU is all you'll need for all but the most powerful (read: that $3000 Uberbox) gaming PC. I recommend the Antec BP550 for both value and effectiveness.
COOLING: Not terribly familiar with this one, but I've heard several people on other forums recommend the Sunbeam CR-CCTF.
And there you have it. :)
"'Casual games' are something the 'Game Industry' invented to explain away the Wii success instead of actually listening or looking at what Nintendo did. There is no 'casual strategy' from Nintendo. 'Accessible strategy', yes, but ‘casual gamers’ is just the 'Game Industry''s polite way of saying what they feel: 'retarded gamers'."
-Sean Malstrom
