Vista will have some plusses (such as Direct X10), but this will be pretty pointless if you don't have a DX10 card. Plus, Vista is more of a processor / memory hog, so unless you upgrade your PC, they'll be a performance hit that way.
AND, the Vista drivers for your hardware will not be as mature as those for XP, so you might encounter more problems at first.
As a general comment, I'm amazed at how everyone is getting so worked up about Vista. Microsoft must be laughing their heads off that people are so desperate to upgrade from XP (which is a pretty good O/S I reckon), even months before Vista is due to be released (late).