If you go to Microsoft's vista website, there is an app you can download and run that analyzes your system (both HW & SW). It'll give you a list of any installed components/apps which are not compatable.
Personally - There's no pressing reason to upgrade if you're happy with XP. Vista has some neat~ish things. But it's also much more resource intensive. It's fine if your system has the muscle. But it'll punish older computers.
just my $0.02