Actually, if it's degrading like that, it kind of make me doubt that the PSU is the problem. A marginal PSU should cause pseudo-random reboots (actually linked to irregular power draws like DVDs spinning up and such). If it can't even get past post, it shouldn't be drawing enough power to make the PSU flake.
My gut-feeling when I see problems like this after a new build is poorly-seated RAM or questionable RAM/Mobo compatibility. Try reseating your memory. If that doesn't work, try other RAM slots or swapping sticks, or only one stick (if you're running two or more; test all combinations). If you manage to get it running, get, burn, boot, and run a copy of memtest86+.
My gut-feeling when I see problems like this after a new build is poorly-seated RAM or questionable RAM/Mobo compatibility. Try reseating your memory. If that doesn't work, try other RAM slots or swapping sticks, or only one stick (if you're running two or more; test all combinations). If you manage to get it running, get, burn, boot, and run a copy of memtest86+.