For your new rig start here:
http://www.tomshardware.com/reviews/best-gaming-cpu,2599.htmlFind the best processor on this list in your price range. Find a cheap motherboard with the matching socket. Figure out if you can afford to get 4gb of RAM that matches the motherboard... if not, 2gb should be adequate for most modern gaming situations (newest games/Crysis might choke a bit). You'll want at least a 500w power supply... but make sure it's a quality PSU (in most cases this is more important than max wattage!). This will help you determine what power supply to get:
http://www.eggxpert.com/forums/thread/323050.aspxYou likely will be wasting money if you get anything higher than a 700w PSU.
What else... oh yeah case. Get what looks appealing and fits your budget. Read reviews before you take the plunge.
Spend whatever is left in your budget to get the best video card you can as this will directly affect your gaming performance. Some good and affordable video cards include the 9600 GT, Radeon 4770, 4850, or if you instead want to play with the big boys get the 5850.