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.