Over the next year Vista will continue to be improved. XP will be phased out eventually.
As far as gaming: Although Windows XP will support DirectX 10, Vista was created with the foundations of DX10 in mind. You can expect the most anticipated games of the year (Crysis, anyone?) to run with more efficiency (it's hard to believe, I know) on Vista. Most of the performance and usability issues will resolve themselves as we near the first service pack. As time moves (far) forward, you'll see less and less support for Windows XP and more for Vista. Current games reportedly do run more efficiently on XP, but, as lesfleanut points out, the difference is insubstantial.
I've been using Windows since the dawn of time. This is how Microsoft's product release cycle work. There's a short "breaking-in" period we all have to endure before we're presented with something functional.
Welcome lesfleanut! Quite a unique name you have there.