Vistas memory management is far superior to XP's.
For example. If you have 2GB of ram, Vista will utilize it to it's fullest extent (XP uses what it needs and then puts it back when done)... Vista may look like it's a memory hog and using too much memory - however it prioritizes where it needs the ram (ie if your playing a game it puts the memory into that .. but if your in the OS just tooling around, it uses more of the ram to make things run smooter).
You can also get a fast USB Flash Drive and use the "ready boost" technology which uses the flash drive for a swap file instead of the hard drive (obviously Flash Ram is a lot faster than a HD).
For gaming, it's not bad. The biggest thing with Vista is the drivers. For the most part, every company now has drivers for their products and if your buying new equipment you shouldn't have any issues finding them. Your biggest concern will be the video card. (Read up on the nVidia 8800 and Vista -- pretty sure they have ironed out the bugs with that but not sure... as for any other video card - no issues).
Also keep in mind for growth -- Vista has DX10, XP doesn't (and probably will never have it). Granted right now there are maybe only 2 games that use DX10 - but it's the growth, it will be there when more games start to use it.
Just make sure your using a newer video card which can support DX10.
So really the choice can still be a flip of the coin, but honestly I have not had any major issues running Vista and playing games.
If I was building new, I'd prolly go w/ Vista just because it's the push the MS is making and it's sure to only get better as time goes on.