I most definitely will vote for ATI Radeon HD 4550, the 4830 looks great, but let me tell you why I still prefer the 4550.
It is passively cooled, and that is a big yes on a HTPC, it is able to offload movie playback of H.264, VC1, and MPEG-2 formats and also is able to send the HD audio formats that are on blue-ray movies over Linear PCM audio on a single HDMI cable.
This one looks great to me.
http://www.newegg.com/Product/Product.aspx?Item=N82E16814131132
And for the record, I think HDMI is awesome, but I can’t wait for the day we don’t have to use cables at all.