SYSTEM USAGE FROM MOST TO LEAST IMPORTANT: gaming, surfing the internet, watching movies
For gaming, the best value cpu right now is the athlon ii x3, as most games only use up to 3 cores. So if the purpose of the build is primarily for gaming, check out the Athlon x3 435 & Gigabyte GA-770TA-UD3 Motherboard. It is around $150 after rebate, and the motherboard has USB 3.0 and SATA 6gb/s.
*EDIT* Mosox the EA650W PSU he selected is not overkill, it's a very good quality PSU that he found with a combo. The Coolermaster 460W is NOT what you would want in a $750 build, it's a low end PSU for people on extreme budgets.
AMD triple-core is the best bang for the buck right now. Some games use 2 cores, very few can use more. The additional core can be used for background tasks.
For a gaming machine with ATI graphics, a 5770 is pretty much as low as you want to go. That should do medium settings at 1920x1nnn, and high settings at lower resolutions. Any of the 46nn or 56nn series cards are going to be very poor at high resolutions, as they're really HTPC graphics cards, not gaming cards.