There's really no need. If given the option of going from XP to either, and having tried both, I'd recommend Win7. On the same computer, boot times run 3.5 to 4.5 minutes for XP and Vista. Win7 takes 45 seconds, and I'm pretty fanatical about keeping the run line in the registry fairly simple for all three. Win7 still likes newer hardware and drivers, but I'll bet it will work fine with less invested than what Vista demands...
Gigabyte G45...E8500...GForce 8800GT... 4 GB of ram... Nothing too expensive!
1) It is more secure, particularly the 64 bit versions.
2) It supports new capabilities like hyperthreading cpu's and trim for SSD's
3) Updated user interface.
4) Ongoing support. XP is close to end of support, Vista will be next.
If you will buy a new OS, get Windows-7, there are no real drawbacks. If you already have vista or XP,
keep on using it until you find something you need in windows-7
@geofelt: XP isn't anywhere near the end of support. As far as I know, there hasn't been a plan released to stop supporting it. I believe the next one to lose support is Windows 2000 (in July I think). Don't forget that most businesses are still using XP, especailly the large clients. XP won't lose support for many years from now.