I got an 8500 gt and I got to tell you it plays movies superbly. I'm waiting on my new quad until I encode my DV videos to DvD so I can't answer that question. Most games play very nicely also. "Not Crysis", but Call of Duty 4 is sweet.
the 2600XT is hands downthe best decoder here in both XP and Vista. I believe THG even did a review of these features and included all the low from 2600XT to the 2400XT/PRO and the 8600GTS to the 8400GS.