Not booting with 4GB RAM

I am running:
Asus P5K-E MOBO
Enermax 500W PSU
Corsair XMS2 4GB (2 x 2GB) DDR2 800 (PC2 6400) RAM

My problem:
Computer doesn't boot when both sticks of RAM are inserted. The computer had been running fine with both sticks before I moved and had the computer shipped out to California. How could a month of inactivity and/or shipping cause the computer to suddenly not run the same?

Should I check BIOS settings in terms of the memory speed and voltage? I am pretty inexperience when it comes to tinkering with RAM BIOS settings.

Thanks in advance,
Kevin

Try booting with each RAM stick individually... If they both boot fine, the problem is likely that during shipping it got bumped around and a stick gotten loosened.