About the only real improvement would likely be that if you dual-booted Vista and XP and used the XP just for games, you could keep it very trim and mean compared to an OS that you use for everything. There are tools that will shut down everything not needed before gaming - iobits makes a good one - but the fact is the more stuff you install under any Windows OS the more crudiffied it becomes. The other good reason to dual boot is that it gives you a good OS to use when something happens to the other installation to fix it, if you're not comfortable doing so with a Linux live CD.
Other than that - excluding the fact that Vista is crudware (*IMO, based on my experiences, YMMV, and I'm sorry if I offend anyone who has had a good experience with it against all odds) there's not a lot to argue for not just running either as your only OS. (again, IMO).
<S>