Plenty of benchmarks here, but they're for gaming:
http://graphics.tomshardware.com/graphic/vga_charts.html
AGP compatibility info here:
http://www.playtool.com/pages/agpcompat/agp.html
While there are a ton of benchmarks for gaming, video
playback articles are a lot thinner. And some of the
articles say things like "we'll have to wait for an
updated driver" and then they don't follow through
with a later article to update the results.
For basic assistance with playback, video cards have had
IDCT (inverse discrete cosine transform) for a number of
years. That means virtually anything at the store has
IDCT. But, more recently, ATI and Nvidia have added
more hardware to assist with video. I think one of
the reasons I haven't been following developments, is
because of the Purevideo results (late and not that
impressive, not fully functional on a couple 6800 cards,
users need to buy a separate DVD player software package).
Here are some links to give you an idea of what ATI and
Nvidia have tried to do recently.
http://www.ati.com/technology/h264.html
http://www.anandtech.com/tradeshows/showdoc.aspx?i=2433
http://www.anandtech.com/video/showdoc.aspx?i=2536&p=4
http://www.nvidia.com/page/purevideo.html
http://www.nvidia.com/page/purevideo_support.html
(Purevideo available on GeForce 6800/6600GT/6600/6200, not 6800GT/Ultra)
http://www.anandtech.com/video/showdoc.aspx?i=2305
(How it is supposed to work.)
http://www6.graphics.tomshardware.com/graphic/20040414/geforce_6800-17.html
The 6200 AGP could well be the cheapest non-gamer card to
get Purevideo playback. As far as I can tell, the 6200 has
Purevideo, but the foolishness at the product launch makes
it really hard to be sure. Who can trust a company that
delivers hardware this way.
http://www.nvidia.com/page/geforce6200_agp.html
This one is $58. The only thing I don't like to see,
is the VGA signals being transferred via a ribbon cable.
If the VGA resolution on your monitor is low enough,
you probably won't care about this. (The higher the
res, like 1600x1200, the more important how the VGA
signals are treated.)
http://www.newegg.com/Product/Product.asp?Item=N82E16814121542
This one is $70 and a full sized card. At least all the
signals stay in the PCB. The product pictures show it
comes with a video output cable (RCA jack) for your TV.
http://www.newegg.com/Product/Product.asp?Item=N82E16814145118
When selecting a card, check the pictures of the product
on Newegg, to see if there are adapters included with the
video card. If the DIN connector to RCA or SVHS adapter
is missing, it is a real pain to find one at retail
for a good price.
Note: You'll be out of pocket another $20 for this, to
complete the package. Why this isn't bundled with the
video card, is just one more of life's mysteries:
http://www.nvidia.com/object/dvd_decoder.html
On the ATI side, Fullstream seems to be a technology for
partially fixing heavily compressed video from the net. The
Avivo stuff seems to be limited to their very latest X1000
series cards (PCI Express only?). Some older ATI cards,
like the 8500, had "Video Immersion" which seems to be IDCT
and some deinterlacing. So, ATI has some baseline capability,
and maybe with the power of your processor, ATI would be enough.
A forum like this might have some useful info:
http://www.avsforum.com/avs-vb/forumdisplay.php?f=76
The comments here are for Linux users mainly, as software
support for Windows will be better. But it does explain
a subtle difference between 5xxx and 6xxx series Nvidia cards.
http://www.linuxis.us/linux/media/howto/linux-htpc/video.html
It is too bad someone couldn't write a quality page like that
for Windows users...
Paul