What computer accessories can I buy to make my Computer a high end Gaming P.C. for around $400/$500?

Well for $400-$500 I would just invest in an entirely new computer. You can only upgrade crap as far as you can, but then you hit the wall of wasting your money vs. how much you can get back out of it if you sell it. Just scrap that old turd, and buy a new computer with Windows 7 already on it and invest in a new PCI-E graphics card.