Graphics card would not be so important when playing ts / mkv files because the are not hardware accelerated. They rely solely on raw CPU power. So CPU is important in his case. If you only play Bluray / HDDVD material, yes, graphics card is important.

But why worry? Just get a ATI 2400 Pro / XT or ATI 3450 and ýou're good to go for Bluray / HDDVD playback for HTPC purposes. If you plan to playback 1080p mkv files, be prepared to have a high end C2D or C2Q CPU in addition to the graphics card suggested above.