I never said it was an extreme case, but we are talking about how a 720W (or w/e) PSU isn't running a 9800GT if the PSU was the problem.
It was never supposed to be adressed as an issue, but talking about tolerance of quality and no name PSUs, and other variables that may apply such as overclocking. All in all, trying to run an expensive PC on a crappy PSU to save $10 isn't going to get you anywhere. It is damn right stupid, and if anything fails, you might bring down your whole system with you.
RE: Yeah, basically to check every signle possibility, attempt to single them out. If you think it is the PSUs problem, put your dad's PSU in your setup, but only CHANGING the PSUs. If it works, than your PSU is screwd. If not, then move onto the motherboard. Try your PSU on your dad's motherboard with the 9800GT plugged in. If that isn't the problem, then back to the whiteboard.
I can't think of any other reason why it wouldn't be working, because it works on your dad's motherboard, and your PSU, and your dad's PSU and your motherboard, but not your PSU and your motherboard. - Such if this actually happens, something is wrong. Your rig might not have enough juice to push out the things attached to your motherboard (if this happens.)
You see, I think it is worth going through all the diagnostic. It isn't a big hassle, taking into consideration the cost of the 9800GT. I don't know the procedure of RMA'ing nor have I done it before, but I guess the diagnostic might be less hassle-some then doing the RMA. You'd definitely have to wait some time before being able to play the games that you can only play on a 9800GT or above, such that the IGP sucks @ss. Nothing against IGP though.