Could be lots of things. We really need to know if you are using onboard video or not. My guess is you're using a graphics card but you are not plugging in the power connectors or the PSUs aren't high enough wattage to run them (or both!) It could be bad or mismatched RAM. RAM can do funny things to video. It could be the RAM is not set up correctly in the BIOS in which case I'd recommend resetting the BIOS using the jumper on the motherboard or trying one stick if there's two.