You are talking about upgrading your computer, perhaps I've missed something but I have no knowledge of your current specs. I assume you are aiming at running Windows XP and shiii+gwx comfortably.
My specs are:
P4 (3Ghz), 1GB Ram, Nvidia 7600GS (256MB)
This runs the game quite comfortably, with the only performance problems when you are with your camera too close to an explosion (dangerous too
).
I also have a slimmed version of Windows XP (SP2), see e.g.
www.bold-fortune.com/forums/ on how to do this. I have used this information in addition to using XPlite.
All Windows are essentially clutter generating OS's. Less Windows is (in my view) always better.
Personally I would avoid Vista altogether, Windows 7 might be a serious option as it promises to be a small footprint OS, but we have to see what that means in MS standards.
[Somewhat off topic]
Directx10 is not downwards compatible with Directx9, but you can run DX9 games with DX10 hardware, and processing DX9 graphics software with DX10 hardware is not very efficient (so I've read somewhere). However, I presume one does not seem to suffer performance loss, on the contrary, which means that there is enormous processing power in DX10 cards that compensates for the inefficient processing sequence, correct me if I'm wrong. In the not too distant future everyone will have DX10 hardware, despite still DX9 games being around, and since there is still no interesting successor for SHIII(GWX)...