Poweredge R300 memory configuration

Hello, currently i have a poweredge r300 configured with 8GB ram (DIMM1 - DIMM2) two ram module each one 4GB, i need to add more memory so i want know if i can add another 2 ram module each one 2GB for have totally 12GB ram, or i must add another 2 ram modules 4GB each one ?
I checked the R300 manual and into the configuration example i can see that for each bank the memory (in GB) must be same.

Thanks in advance