For media box/HTPC use, hadware video decoding (nVidia VDPAU, ATI/AMD XvBA) is a good thing to have. nVidia VDPAU is better than AMD/ATI XvBA.
The ATI/AMD cards will still work, but its more likely the decoding will have tobe done by the CPU. For AMD/ATI cards, HD5450/6450 is what I would get.
With VDPAU there are various 'feature sets', as you can see here-
nVidia G210 has the same feature set as all the other GT2XX cards (and GT4XX cards as well) and its cheaper/lower power use than the other GT2XX cards as well. So for the cheap nVidia cards, G210 is the way to go IMO.
GT520 is newer, uses a tiny bit less power than the G210, and is meant to have better support for H.264 and MPEG2. Might well be worth the extra $5-10 over a G210 (obviously prices will vary, thats just the current pricing at the store I use most).