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.
I live near a microcenter so at the time the mobo and i5 2500k combo was cheaper than an i3 2120 and mobo from newegg. I was not going to overclock but i figured it was nice for future use so i could extend computers lifetime. I have tried to find amp ususage needed for video cards because i know that is a more important than just total wattage of the unit (you get what you pay for). Doing the extreme power supply calculator i was getting 347 watts with 90% usage...figure i will not do that most htpc usuage and then at night when the kids are in bed i get to play some games. Diablo 3 and then some FPS...just getting back into this.