ATI HD4670 seems to get recommended a lot, that will be my next htpc card.
i heard that 4550 has some issues with windows7 + deinterlacing, which is why some people have been saying go for 4670
that is true. here in NZ cheapest 4550 is $90 and 4670 is $140 so 50% extra price.
if you compare performance it is about 100% better
chart_54_wic_1920_trilinear Image / Picture
altho of course that is not necessarily relevant to htpc
however i would personally spend the little bit extra