Windows 7 pro will run in XP mode,
link.
Get 7 Pro, which is exactly what I will get on my next computer.
I had my first experience with windows 7 yesterday as my wife's computer (XP Pro) died and I bought her an on special laptop with windows 7 home on it. I must say it was very easy to use and ran very well, loaded all the old files (I managed to recover) and the old email applications I put on it (even though it was for XP). I am an old windows user, 3.1, 98, 2000, Xp pre, and also Mac's through the years, and there is almost a bit of a mac feel about windows 7.
Alan.