There is a considerable difference in performance. That said, I went for the AIW 7500 for half the price, I guess I just can't see buying the latest gen card knowing it will be cheaper in a couple months.
Bottom line:
If you need the best frame rates in the newest games or can use the firewire port...get the 8500dv
If you need a great gaming card with an excellent feature set at a great price point...get the AIW 7500.
I'm a happy camper with mine, currently running 6052 drivers w/MMC 7.6, about 5200 on 3dmark 2001SE, capture highest quality mpeg-2 IBP frames without dropping frames, play all my games in high resolution ( 1280X960 or better) works for me.