Wow this thread went places while I was working this afternoon.
Windows - do not buy Vista 64. Pointless at the moment, Win7 will be released in one month and should cost about the same, just wait for it (unless you can get a copy early/cheap with a .edu email).
RAM - just buy two more gigs of the same speed you have now. Anything faster than DDR2-800 is pretty pointless on C2D setup, especially the e4x00 series which feature the 200mhz fsb. Trick here is just keep it cheap, save your money toward a full upgrade. And only buy additional RAM if you also plan to OC your CPU to ~3GHz - otherwise you're better off just biting the bullet and upgrading the innards now.
GPU - 8800GTS probably acceptable for now, ATI is supposed to be releasing their 5870/5850 tomorrow and it's expected to kick some serious ass. Hold out for a few months and see nVidia's response and for pricing to settle down before buying a replacement for your 8800GTS.
Overclocking your e4500 should net you a bit more performance from your 8800GTS, adding the 2GB will help smooth everything out a bit more. You could even grab a single 1GB stick since you're on a 32-bit OS to save a few bucks.
If you decide to go the major upgrade route I'd suggest an i5 750, 4GB DDR3-1333/1600 and a solid P55 motherboard - should be able to get out for around $350 or so for these parts. You would see a moderate increase in gaming from this upgrade but honestly not much more than just dropping in extra RAM and overclocking your current CPU. GPU has more impact on gaming performance as long as you have enough CPU to push it (which is where you're a bit short right now - but OC to 3GHz and you're gtg for a while). But by the same note you want more than a 3GHz C2D to push a 5870-level card - the trick is to keep things balanced.