anything but home basic or starter. that's just my 2 cents.
ultimate is probably best, since it has all the features, and it can support a dual CPU (two actual CPUs) and ridiculous amounts of RAM. you can build on heck of a computer with ultimate. but there's nothing wrong with Pro/Business/Home Premium either.
Call me cynical but surely the windows vista basic is probably best? Seeing as it reduces the amount of system memory by taking away features which would otherwise clog up memory and processing power? It still supports 64bit and directx 10, so there are no theoretical limits to the software as suggested.