The 8800 GTX should be good for a while as far as gaming performance goes. What resolution do you run games at?
When you do get vista, go with the 64-bit Premium. (Or wait for Windows 7)
The framerate drop in GTA4 is probably due to your processor. I would drop a new heatsink on it and overclock it. You should be able to get it to 3ghz easily. You could even do a modest overclock with the stock heatsink.
While you may need a BIOS update, you could upgrade the processor to either a Dual-Core E8400 3.0GHz (1333MHz FSB) or a Quad-Core Q6600 2.4GHz (1066MHz FSB) or over-clock your current processor as Irkjab suggests.
Well, the 8800 GTX might have some trouble at high settings on that resolution.
You could probably get a 3ghz overclock without any voltage increase. To do this, you would go into the bios, change the cpu frequency from 266 to 333, and change the cpu to ram frequency ratio to such that your ram runs at 800mhz. The specifics of how you do this depends on the bios though.
If I attempt to overclock, is there any thing I should look out for precautionary wise? Is it usually trial and error when it comes to overclock? and also am I supposed to test my overclock? If so, how?