My work laptop had XP SP3 on it, but a software update made it behave very weird and slow, so I wiped it clean and installed Windows 7 Ultimate on it.....and now it flies!
The laptop specs are hardly what you could call "ultimate"....Celeron 1.86Ghz, 1GB RAM....but somehow it's blindingly fast. Love it.
Sorry, but it is a silly question. Win 7 is the latest technology and supports the latest hardware. Unless you are a Luddite, it would be stupid not to upgrade when you can. The only exception would be to support software or hardware that is not yet compatible with Win 7.