XP is definatly going to hold you back. Win7 is solid and usable. I always use Ultimate, but Pro should be fine.

You'll also want to try for at least 16GB of RAM.

My answer was to make an M3/V3 clone at 2600 Polys. :-)

mo·nop·o·ly  [muh-nop-uh-lee]
noun, plural mo·nop·o·lies.
1. exclusive control of a commodity or service in a particular market,
or a control that makes possible the manipulation of prices