Please recommend Processor and Graphics card(s)

Basically I want to get the most out of this computer. I've posted another thread about some awkward problems I'm having, and since no one seems to know anything to solve the problem, I've decided to replace every possible non-working part.

I've already replaced the motherboard, and am now running an Asus P5N-D with a 600W power supply, 2x 18A,12V rails. Basically I'd like someone to recommend the best possible processor and graphics card set up for this motherboard under $1000. I am running Windows XP, and want to get the best possible gaming performance out of this machine. If there's a strong consensus for more pricey alternatives, I am willing to consider them, but of course I'm not made of money and would prefer the less expensive options.