Click the following link for a comparision of several video cards (the review itself is for the HD 4550).http://www.xbitlabs.com/articles/video/display/ati-radeon-hd4550_6.html#sect0
I haven't read the article but based on the matrix, I would recommend the HD 4650. The HD 4670 uses about 47w of power, so I imagine the HD 4650 should be around 40w - 45w.
The clear winner in power consumption is the HD 4550 at 18.5w, it did not score as high as the HD 46xx / GeForce 9600 GT (61w) which both tied in first place.
I have a passively cooled 9600 GT made by ECS (??) in my HTPC. I probably would have bought the HD 4670 had AMD released it when I was fishing around for a video card.