OK If you live in the USA, that means you can order from newegg.

CPU: [url=http://"http://www.newegg.com/Product/Product.aspx?Item=N82E16819103644"]AMD Phenom II X4 940[/url] or [url=http://"http://www.newegg.com/Product/Product.aspx?Item=N82E16819103703"]Athlon II X4 630[/url]
I have chosen cpu's without tock heatsinks on purpose cause they suck anyway. So you need to choose one that fits AM3 socket. [url=http://"http://www.newegg.com/Product/Product.aspx?Item=N82E16835181011"]This[/url] one is nice for example, but you can choose a cheaper one if you want.

The difference in cpu speed in mhz can be easily helped by overclocking. But both cpu at same speeds, then the Phenom will be slightly faster due to the L3 cache. For gaming, this is better (not by much). It is possible to unlock the cache from Athlons II X4's, but this is no guarantee and based on luck.

Motherboard: any with 790FX, 880G, 890GX or 890FX will do from a decent brand. It's up to your wishes, you cant ATX or the smaller mATX, Need lots of Sata ports or not, Need USB 3.0 or not, looks, .. etc etc.

RAM: Choose a motherboard with DDR3, Any 1600mhz (or more) will do with decent latency. (avoid CL9 for AMD systems). [url=http://"http://www.newegg.com/Product/Product.aspx?Item=N82E16820231303"]These[/url] are good and very cheap. [url=http://"http://www.newegg.com/Product/Product.aspx?Item=N82E16820231321"]These[/url] work at extreme low voltages for energy savings/better overclocking.

Most important for gaming now:
GPU: Personnaly I'd choose an ATI card, NVidia is a bit overpriced atm. Best bang for the buck are ATI cards.
HD 5770 and HD 6850 are very good and cheap. If you want really high performance get a 5850/5870 or the new but out of your budget 6970. Also, if you game at a very high resolution you MUST get a 5850 or higher. If you game at 1680x1050 a 5770 will do fine.

For Hard drive, PSU, DVD drive, Case: You probably can recuperate from your current system. If your PSU is not good enough I'd recommend to buy a new one. For a mid to high end system like I recommended a 500W from an A-Brand will do!

I don't know if you also need a new monitor, I hope not because your budget is quite tight.

Try to assemble something on newegg and post you shopping cart here.