But some games like Just Cause 2 don't support that anymore. To run 64-bit and game I'd wanna have 4GB of total RAM or more.
so you really need to do all of that at some point. The best thing to do would be to buy a overclocking heatsink and overclock that CPU then buy something like a HD 4890. This should do the trick till you do the rest.
If you're a student you could hit up win741.com for a $65 copy.
Priority upgrade in my opinion :
Get 4GB RAM and Win7 x64
E6750 and 8800GT can still perform decently.
Upgrading to any LGA775 quads won't make much difference.
Upgrading to HD5850...your proc will be the bottle neck for the GPU.
Replacing the proc to i5, i7 or X4 ...you will need to buy a new mobo...and new DDR3 RAM kits...
Too much investment at this point...
Save the money and upgrade the proc, mobo, and RAM including the GPU next year.