I have the same problem with my computer. Will only boot with 1 stick of RAM in. I got around it by clearing the CMOS several times, just to make sure it was well and truely fresh. Then I put one stick in, booted, went straight into the BIOS, set the memory voltage to the required amount, save, turn the computer off as well as the power to the PC. Put the other sticks in, and try booting. This usually works, sometimes it randomly decides it's not going to boot. I found it was something to do with the voltage regulation on the mobo - the "auto" option would just cause my system not to post.