There's an option in the WMC menu that lets it start on boot, I think you've probably seen it now.
The PSU has given me no problems at all, seems plenty powerful for what I've put in, the Athlon X2 is fairly low power as is the board, no real expansion cards bar the TV card and I fully expect it to handle a Radeon 4350 with that too. It has a lot of unused cables in mine though, so you have to find somewhere to tuck them, which isn't too hard. I do wonder why though, since you can only really install one HDD and one optical drive in there. Still, nice to have extras.