I personally wouldn't put it as clear cut as the above for XP. If it's a case of Vista then sure, switch straight off - 7 is basically what Vista should have been in the first place, far far better.
If you're planning to switch from XP and you've got a system with fairly modest specs though - my advice would be to stick with it. XP's requirements are far less than Vista and 7 and as such you may find your system doesn't perform quite as well. I disagree with 7 being far more stable than XP ever was - XP is a rather mature OS now, has gone through a lot of security and stability fixes and as such is rather stable. I'm not saying it's unstable, but 7 just hasn't been around long enough for anyone to make that comparison fairly.