I recently put together an HTPC, to be used for playing HD tv (via cable & internet), recording TV, and playing movies & music.
Currently, I am not using a discrete graphics card, and instead relying on the CPU/chipset graphics solution. Unfortunately, I'm not too thrilled with the performance of this - HD images occasionaly look "bad"(e.g., sometimes I see coarse gradients between colors, and sometimes images are rendered in a "jerky" manner, particularly when watching fast-moving images like live sports).
Generally speaking, for DVD playback quality the HD 55xx scores near the top for AMD (ATI) cards, but nVidia cards scored slightly higher. For Blu-Ray playback quality AMD card trumps nVidia cards by a pretty good margin and the HD 55xx score the highest along side the more powerful HD 5670.