No Display after installing new MB

I installed this motherboard: http://www.digital-daily.com/motherboard/asus-p4p800-e/
Into this PC: http://reviews.cnet.com/HP_Pavilion_a300n_C_2_6_GHz/4507-3118_7-30574458.html?tag=nav

Took the hdd/cpu/memory/video card(BFG 5200 pcI) from the PC with the new motherboard and put it into another case with a new 500W power supply.

After everything was done I am able to boot the computer up, but there is no display. I called support and they said it was either the power supply/CPU/ or the motherboard.

So the computer boots up but the monitor says PC entering power saving mode and goes into standby.

Yeah...

 

Video card is not properly seated.

 

double and triple check that everything you put in is installed correctly and properly seated, incl. memory and cpu

 

Memory not seated correctly has always caused me the most trouble, and so easy to check too.

Sometimes a bad CD played will cause trouble. Try unpugging everything except the bare necessities, power up. Add something else, power up. Until either you found the bad device or everything has been added.

 

Everything looks like it should switch and work, my only thought is did you hook up the p4 plug on the mother board? this is the little 4 wire square plug, you can only plug it in one way. ed

 

I am sure that everything is properly seated. All parts I'm sure work except for the motherboard as I've tested them out in another computer. The CPU is s478 and the mb is s478 but could there still be a possible issue with the CPU?