Only nvidia card I've had is tnt2 ultra (I liked it fine).
I've had AIW Radeon since it first came out. Never ever had any problems that a driver update didn't fix, at least for me (forced to update the driver 1 x only due to TV problems with initial drivers, & ever since then only updated the driver&MMC 1 more time so I can try the new mmc 7.5) And I have the a setup that "supposedly" has major problems with Radeons:
kt7a raid, tb800, sb live, sony trinitron flat tube kds avitron 19in... All known to have prob with radeon(at least in the past), but not me.
Using my comp as the center for my home entertainment system & the only graphics card that I would buy in the future would be AIW series unless nvidia comes out with something better that works better than their "personalcinema crap" that I tried out briefly & unsuccessfully cause it didn't work properly.
GF4 sounds mighty impressive though... hope they implement their personalcinema better with that card than in the past.
If price is no object, I really liked what the reviewers are saying about GF4, but 8500 is no slouch & I guess you have to ask yourself, is the price difference worth it? (however much it is for you)