the Gigabyte 7600GT is a solid choice, however a 7900GT is leaps and bounds faster, and Im hoping silent ones will come out soon
I dont recommend the Asus Silent 7800, despite it being marginally faster than the Gigabyte 7600, it is more costly, and because its on a larger die process than the 7600/7900 it runs quite a bit hotter.
Truthfully, if you can, wait for the 7900GT with HDMI that is supposed to be released this month. That will be awesome for an HTPC.