anandtech just put up a HTPC gpu roundup for you, including the ati 6450, 6570 and nvidia 430 and 520 cards http://www.anandtech.com/show/4380/discrete-htpc-gpus-shootout/1
geforce gt430 is a good choice for a htpc nvidia's control panel is a bit better than ati's, i always forget where their overscan settings are. check amazon there is an asus card $60 plus $20 rebate if you want to bother