ckfordy - My 7800 had input capabilties, and I'm pretty sure all the others do... I mean I did have one of the cheapest ones.
3 are Component out, one of those is plain RCA out, SVHS out, then RCA in and SVHS in. They all should be labled something like this, component being like Rb and other rather random letters
Look on nvidia's site, they have drivers for the input somewhere there. After that, you just need the correct software. Media Player Classic will play the video back no problem, but will not capture anything...
A side note, as you might guess, the card does not have audio capture. You must use the line-in on your sound card to get audio.