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You should really run this on your existing system to see what needs to be "fixed" before you upgrade. It also gives you an outline of what the newer editions provide and don't. For instance, since I was running XPProSP2 it said that the Basic Home version would result in lost functionality because some things included in XP just weren't included in the Basic Home version. That's a little ridiculous to me, but there ya go.

http://www.microsoft.com/windows/products/windowsvista/buyorupgrade/upgradeadvisor.mspx