I recently built a new system with a Asus M4A78 MB. When i boot up the system with all 4 2gb ddr2-800 sticks in the system freezes during the bios screen. When i took all but one of the sticks out and left the one in slot 1 it booted up fine and i was able to install the OS. I then updated the BIOS from the ASUS utility. Then I added a second stick with no problem to slot 3. I have tested the other two sticks and I don't think they are the problem. Is my MB bad? Do I need to set do something different with the BIOS?
Here is my system
I'd guess it's a voltage problem, based on the fact that the system works fine with two sticks but not four, and you haven't changed the settings.
When you fill all four slots, RAM will often require slightly more voltage than spec to run properly. So it sounds like with two sticks, it's getting the 1.8V it needs, but with four, the system is autodetecting the standard 1.8V correctly, but not realizing that it really needs to be supplying more than that. Basically, it's aiming right, but the target has moved.
Fortunately, if that's what the problem is, it's a simple fix:
1) Remove all but two sticks of RAM so you can boot again
2) Go into the BIOS and manually set the RAM voltage to 1.9V
3) Put all four sticks back in
4) Hopefully it should work now.