Follow up:
Took me a few days to get to this point, but I finally have a working computer!
The actual problem was the motherboard we had gotten. After trying different power supplies, we still couldn't get the board to power up.
We ordered a new board and the 400w power supply worked without any problems at all.
If I hadn't checked with you guys on the trimming, I'd probably would have been stucking buying another one. Thanks!
