Here are the vdpau supported cards:
Note that, as cehoyos pointed out, the 8400 with g98 chip supports up to vdpau set B. This is in case that you buy a g98 (older versions might support set A, but i can tell you that its not enough). Some HD movies might not work correctly with vdpau (if at all) with older chip versions.
So, find a nice passively cooled gf 210 and you are set (i have an ASUS version with fan and its a bit noisy). You cannot go wrong with this one - it played all HD .mkv's i threw at it so far. Just make sure you get the vdpau-enablem smplayer/mplayer.