If you must add a GPU, I can't think of a more practical choice than a gt 430 for XBMC. I currently have a passive one, From Zotac. It's small and of course silent. I play full Bluray rips, not encoded, using Makemkv, that means 25 to 50 gb rips over a GB Ethernet without issues and using dxva in xbmc. Bought it at ncix.com a long time ago. Asus makes one too, passively cooled I mean, no fan if you can't find Zotac. There are variations for low profile as well.
Edit: if I was in the market for a new GPU, say my current gt 430 died, I would actually move up to a GT 640. Just like the gt 430 does not require external power, supports all the bitsreaming audio we want and deserve, is way more powerful than the gt 430, run cool and quiet although I am not sure if there is a passively cooled one yet. Anyhow if you dont mind spending a few more $$ you cannot go wrong with the GT 640 or the 430 for a few bucks less.