I may have a line on a legitimate free copy of win7pro 64 bit, and I think I'll start shopping video cards to start with.
Sorry for the diversion into the theoretical.
I agree, a video card is your best candidate for a best bang for the buck upgrade, even more so than an OS change (although if it's free, it doesn't count.) I might suggest that you do not limit yourself to Nvidia cards, ATI was generally the best bang for the buck last I checked once you move away from the low-end. The 5770 is a pretty decent performer for around $150 and it's hard to go wrong with the 5850 if you want to spend another hundred bucks or so.
What I'd suggest doing is determining what your budget is, going here
http://www.tomshardware.com/charts/2010-gaming-graphics-charts-high-quality/benchmarks,111.htmlpicking the resolution you want to run at, and whether or not you feel strongly about antialiasing, and use the "Sum of FPS Benchmarks" at whatever is closest to your resolution and AA choices.
If your not sure, I'd suggest you use
Sum of FPS Benchmarks 1920x1200 with anti aliasing, 4AA (High Quality) Then, look at how the card choices in your budget range compare in the benchmark vs their cost. Make sure you realize that CF and SLI means more than one card, and not to compare those results!
Also take note that unless you do upgrade OS to 64 bit, anything over a 1GB card is going to start drastically affecting the amount of memory XP has available, so bear that in mind as you move up the scale to cards with more than 1GB of RAM.
<S>