Microsoft realised what people were doing with the corporate versions, so they're changing the licensing for them apparently.
As for the versions, go to http://www.microsoft.com/windowsvista/getready/editions/default.mspx and you'll no doubt see that the Ultimate edition is pretty much all you'll want. The home edition doesn't fully support all of DX10's features if I'm correct. Can't be bothered to find the page on the net where I read that, but plump for Ultimate.
Unfortunately the upgrades don't do Ultimate - you can purchase an OEM of XP Home, Pro or MCE and get Basic, Business or Premium. Check out the online stores for exact details, though last time I checked with aria, they weren't 100% sure how the upgrades would work. I guess just the same as the current upgrades, but you'll have to mail-in a voucher to receive your copy - which according to Dell costs between £11-25.
No need to upgrade me computay for Vista. Everything had Vista ready logos on them. I'll possibly go for an 8 series GPU, only when this starts to slow...