The 5970 is indeed the best single card, but unless your editing applications can utilize GPU power (pretty much all of them can't), it's money wasted.
The Quadro series are professional graphics cards for workstations - things like Solidworks, CAD, 3D modeling applications, Matlab, etc. That's why they're expensive. They make poor companions for gaming or media-based PCs.
Nvidia also has a consumer series just like ATI does (ATI also has a workstation series). You should, as mfarrukh said, be looking for a lower-midrange card. The only work your card will be doing is accelerating playback, really, which isn't very tough to do. A 5750 or an nvidia 250 will be more than enough for you. As he/she also mentioned, you don't need $3000 for a good editing PC...$1000 will be more than enough.