Im just talking about the processor. I know that I will have to get the other upgrades. Sorry if i wasn't clear enough.
If $500 is the cash that you are putting aside for the processor only, and you have more cash to use to purchase other parts, then the i7 920 is your best bet. It overclocks easily to i7 975 levels and beyond, and will game and run any apps you care to throw at it. Pair it with a nice motherboard (P6T Deluxe) some good quality RAM (6Gb MHz corsair will do) a decent graphics card (5850/5870 for DX11, 4870/GTX275 for DX10) and a nice PSU (corsair, PC power and cooling) and you'll be blown away by the performance.