I'm doing the same thing, buying an i7 build for video editing, etc.
One thing I've noticed is that for any GPU acceleration that is available, like some of my plug-ins in Sony Vegas can be GPU accelerated... that Nvidia cards >>>> ATi
Don't exactly know why, but it's just how it is. So I'd actually highly recommend a GTX 260 for you.
I have a 9800GT right now that I'll be using, and you might want to look at that option too. It's a good deal less powerful, but still a decent, modern card, that can play all games, albeit maybe not at high settings.