Are your memory slots color coded?
Some mobo's have slot 1 and 3 as the matching slots.
Let's backup for a minute and look at everything.
Disconnect everything from the Motherboard except
- video output
- 20+4 powercable
- 4/8 pin 12v wire both coming from the powersupply,
- Cpu fan wire
- power and reset button to the case
- case speaker
The goal here is just to test the mobo:
If the computer still will not boot up the please remove the motherboard from the computer along with the power supply
place the motherboard on a piece of card board larger than the motherboard,
this will eliminate a short from the mobo to the case which could be a possibility
Install the cpu with, 1 stick ram in dimm 1, power supply, case switch and case speaker
Connect ps2 mouse and keyboard along with the monitor
Repeat the above and power on
If the computer now boots into bios you most likely had a case short so make sure when installing the motherboard in the case that you use standoffs and they line up with the mounting holes in the motherboard and none of the standoffs touch anything else on the underside of the board.