Actually I am not an expert in any way, shape, or form, but in terms of options, here's the way I see it.
The HD 3850 is the safest option for futureproofing. It also seems to have slightly better decoding and post-processing. Downside is it's $170.
The 8500GT is a great card for HD video but is crippled in every other aspect, which is to be expected at $70. I don't see any significant advances in video encoding in the near future that would render this card useless, but there's definitely no guarantee there.
The 8600GT (DDR3 version) is a decent middle ground for $100...same hardware acceleration to help with HD decoding, but it has a little extra power if you want some insurance down the line. ATI's 2600XT is a similarly-priced alternative.
I'm personally leaning towards the 8600GT, but since if you're actually relying on my opinion, I will recommend the HD 3850 to cover my butt. Also I've heard reports that the 8600GT's post-processing features aren't available on Windows XP, so you might want to take that into account as well.