For a while the RAM was not being manufactured. I recently purchased 8GB modules from memory-up.com (it is black diamond memory), and they work fine. Earlier modules I ordered did not work, they need to have the x8 configuration (whatever that means).
My server came with 4 GB ram (2x2gb). I was told by dell technical support to install the 8 GB modules in channel 1 (the white tabs) and the 2GB modules in channel 2 (the black tabs). It also seemed to work by mixing the 8GB and 2GB modules in the channels also. In the end I did as suggested by technical support.
When mixing the ram sizes I got an error message: dell unsupported memory configuration dimm mismatch across slots detected. By pushing F1 it worked with no problem at all. In the BIOS, I disabled the F1/F2 error setting, and that allowed the computer to boot without waiting for the F1 press to continue. Admittedly it is probably not a good thing to ignore error messages. If not mixing the RAM, I did not get the error messages. Dell technical support didn't have an answer other than it is non-Dell memory.
My server is running Centos 6.0 (a RedHat clone).