When adding memory to DDR800 mobos, often you have to raise the VDIMM by .1V first. Try raising from 1.8 to 1.9Vdimm in the BIOS, before you add the extra two modules, then boot up.
If it isn't booting up at all, the CMOS BIOS probably got corrupt when booting with 4 modules. Reset the CMOS BIOS by removing the CMOS battery for a few minutes, then replace battery, boot to BIOS with original two modules, and set the BIOS to Optimum and reboot.
After a clean reboot, raise the Vdimm by .1V, prior to reinstalling the two new memory modules.