<< but what exactly is so great about the DV features vs the TV card? >>
Adaptive deinterlacing, temporal filtering, and video gamma enhancement , firewire support for digital capture, TV-out resolutions to 1024x768, MMC 7.5, FM remote control, Hydravision, DVI-I, Hardware support for Component video output via DVI-I to YPrPb dongle(not yet available), Dolby digital stereo audio output (S/PDIF), saves at least 1 PCI slot (2 if you consider the firewire)
In other words, the 8500dv is a complete, mature, package of the latest ATI features mated to the core of their top consumer level graphics card. The TV Wonder however, is unfortunately a mediocre product based on an outdated chip with limited capability and half-hearted driver support in an NT environment, which IMHO is critical for PC video work. If you can get it to work, and find MMC7.5, it will do the job, perhaps very well. As long as you know that going in, thats fine.
Personally, I've had great luck with my AIW cards and would have gotten the 8500dv rather than my current AIW 7500 if I had a use for the firewire support, and would recommend them over any AIW type card. Hopefully nVidia will get serious with Personal Cinema, but they have a long way to go. The TV Wonder is a different story however. Hauppage and Avermedia also make TV capture cards that are worth a look, but beware that none of the capture cards are issue free. Your best bet is to determine exactly what you want to do with your capture card and buy the card that best fits your needs.
If you don't use the features, AIW cards are expensive, but if you do use them, they are a real bargain IMHO.