CM2X1024-6400 help going to "4-up"

I've been running my system

Biostar TForce 6100 AM2

Athlon 64 X2 3600+

2GB CM2X1024-6400 (2x 1GB)

I wanted to add 2 more GB RAM (want to upgrade to Win 7 x64) so I ordered another TWIN2X2048-6400.

I notice that my old RAM is 5-5-5-12 and the new RAM is 5-5-5-18.

I can run either set of 2gb but when I add all 4 sticks, the computer won't boot at all.

I know that different memory settings are recommended for 4x sticks of RAM, but I'm not sure what setting to use. I realize 2x 2GB sticks would be better, but my motherboard manual mentions that it cannot accept RAM more than 1GB per slot.

If I need to change RAM settings in the BIOS, can I do that with only 2 GB installed?- as I can't even get to the bios settings with all 4 sticks in.

What should I do?

Thanks in advance