ideal HTPC cards are low-powered, low-heat, low-noise (preferably with passive heatsink), and low-profile (to fit slim cases)..
ATI HD5450 LP: ~6W idle, 19W max
ATI HD5570 LP: ~10W idle, 43W max
ATI HD6450 LP: ~9W idle, 27W max
ATI HD6570 LP: ~11W idle, 60W max
NVIDIA G210 LP: ~9W idle, 31W max
NVIDIA GT430 LP: ~49W max
NVIDIA GT440 LP: ~7W idle, 65W max
NVIDIA GT520 LP: ~29W max
compared with gaming cards..
ATI HD5770: ~18W idle, 108W max
ATI HD6870: ~17W idle, 163W max
ATI HD6970: ~22W idle, 287W max
NVIDIA GTX460 (768M): ~14W idle, 146W max
NVIDIA GTX560 Ti: ~14W idle, 202W max
NVIDIA GTX580: ~33W idle, 317W max