personally, I'd just do as you planed, get some more DDR2 rams, add another 2x2gb will get you to 6gb should be enough, then get win7 x64 bit, overclock the cpu to 3,2ghz. you may need a new heatsink depending on what heastsink you got now. corsiar 520W should be good enough.
one last upgrade is the g-card, but that I think you should wait until you try this game to see if your 8800gt is enough, if not, get a new graphics card then.