Well I assume you read what I said in the GPU as a second processor thread since you posted after me, so that would be one thing and it is exclusive to DX9 and up cards (although it can be done with any DX9 card, even the non AIWs or an nvidia).
Other then that there are small differences between the AIW cards; as you mentioned the AIW 8500s have a firewire port, the AIW8500s have a digital TV tuner(not for use with digital cable... it is supposed to tune in pictures faster and have a few minor benefits but the AIW9700 went back to an analog tuner since the digital one ran too hot), all of the old AIW cards used the original rage theater chip while the AIW 9700 and AIW 9000 use the theater 200 chip, the AIW 9700 (and 9000?) have a built in hardware MPEG encoder/decoder called the "cobra" and the AIW 8500 and better have something called video soap which helps to clean up cable feeds by applying fiilters in real time (see that anandtech review I posted in the other thread for good pictures of what it does).
Make no mistake about it the newer AIW's have a few small benefits over the old ones but the main reason to get an AIW 8500 or AIW 9700 over an AIW 7500 is gaming performance, there is no signifigant video editing reason to do so unless the DX9 pixel shaders being used to add effects in real time is a must have (and you have the software to do it).