Windows 7 Ultimate...
It has the word "Ultimate" so it must be the best and fastest.
Dude, just go with what you are most comfortable with or what offers you the best features.
- Got Windows XP already and wanna save some money? ==>> Stick with XP.
- Yer a gamer and you want DX10 / DX11? ==>> Go with Win 7.
- Running a lot of 32-bit business related software in Win XP? ==>> Win XP
- Running a lot of 32-bit business related software, but want a newer / prettier interface? ==>> Win 7 Pro so you can up the Win XP Mode application (not really for games)