Most of what the replies have said are accurate. To add my 2 cents (I've built many computer for friend and family, and just recently dropped $800 for my brother's computer that he plays wow and tor on.
Processor: i5-2500K is the sweet spot, i7 can certainly yield better results, but 100% not necessary
Hard drive: stick with any basic 7200rpm drive if you're on a budget
Memory: I can dual box TOR on max settings (including shadows) on 1920x1080 and quad box wow on ultra with full AA and shadows with 4 GB @ 1333 MHz. I don't know a single hardcore gamer (and I've played on the MLG circuit) who are gung ho about more than 4GB of RAM. Believe me, 4GB is MORE THAN ENOUGH to have [primary game] + [FRAPS recording] + [10 tabs open in the background] + [music playing from internet or itunes]. This is where you can save money, but if the difference is negligible, more can't hurt for future proofing.
Graphics card: Been using NVIDIA my entire life, but ATI is where the value is. I'd settle for any card in the 200-300 price range. Google around, any "budget" card in that price range is going to be fine.
Power supply: as long as it has enough wattage (and a little headroom) for whatever the min. for your graphics card is.
motherboard: as long as it's compatible with your cpu/ram (LGA 1155)
Everything else doesn't really matter.
