There's no requirement to move to Vista yet unless you need to. There's certainly no harm in doing so if you want, but as with all OS changes, research compatibility and features carefully first.
Compare your current features of XP and choose a version of Vista that has the same, if not more. Also, check your current software and hardware is compatible with Vista. I think Microsoft have a compatibility tool to check?
If possible, I wouldn't jump in the deep end, even if Vista is expected to work perfectly. Try and make it easy for yourself to switch back to XP if Vista doesn't work out. E.g, install Vista on another partition and dual boot into it. Or, Ghost your PC with XP on and keep the image safe. Install Vista and have a play.
Don't activate it straight away, test it thoroughly first. If it's all fine, activate. If not, don't activate it and go back to XP.