weirdest computer problem I ever had

I just built a computer with these specs:

intel core 2 duo 2.4ghz E6600
4gb ddr2 (1gb x 4)
250gb hard drive
asus p5B-E
Evga geforce 8800gts
antec 400 watt power supply

I don't get a post or bios or anything on the monitor.

Somebody on another forum said it was most likely my memory, and after doing research, people where saying that the memory wasn't capatable, so I got 2gb more, and then I found out all 4gb are capatable with the motherboard.
The cpu was fried, so I sent it back to intel for a replacement, I tried 3 different video cards, and 2 different power supplies, but still have no post.
I am thinking its the motherboard, but why does the computer turn on? The longest I had the computer on was 1 hour just to see if it would reboot or anything, but of course it didn't. When I press the restart button, the computer shuts off, and then turns back on, so I am thinking its the motherboard, but I need a few more tips.

I know that a 400 watt power supply shouldn't be enough, but it is. I had a 700 watt, but that didn't make a difference, so I sold it.
answers would really be nice.