Nvidia’s PureVideo is freakin awesome when playing back MPEG-2 content (DVDs or recorded tv) and does hardware accelleration of HDTV WMVs, but not every 6×00 supports it. Without hardware accelleration, a 1080i video stream would use 90% of an Athlon64 3000+. With PureVideo hardware accelleration on, the CPU was around 15%. You HAVE to use nVidia’s DVD decoder for this to work, and download a patch from nVidia.
If you have AGP, then the 6800, 6800GT do not have full PureVideo support. All 6600 cards do. If you have PCIe, then the 6800 series have full PureVideo support, as well as the 6600 series. 7800 definately has full PureVideo support, but only comes in PCIe. Not everyone can get PureVideo to work with all motherboards. I had limited success with a 6600 on an AMD-64 so I eventually went with an ATI 9800, which has good support, but does seem to have interlace artifacts alot.
When you can get it to work, a 6600GT. The picture is awesome, but your mileage may vary depending on your motherboard.
Just my 2 cents.