I don't think 400W is going to cut it, especially if you're going to overclock your CPU (which I assume you will since you have unlocked i5). AMD recommends at least 500W for HD 7850. It doesn't use much power, though, but it's good to be on the safe side of things.
I have HD 7850 personally and I'm very satisfied with it, but it's probably overkill for HTPC. However, if you plan to game as well, then it should be fine.
Just for reference, it can run BF3 on ultra 1680x1050 with steady 60-65 FPS.