get cpu-Z and check the timings of both DIMMS (you'll need to install windows for this). Boot up with your first DIMM, check the timing, and write it down. Turn off the machine, stick the 2nd DIMM in the slot, boot up, check the timings. For you to use both DIMMS at the same time they need to match.
Even if you bought a matched pair of DIMMs, you should still check the timings. I bought a matched pair of ram that didn't match.
Btw, you people should stop blaming the board. Most of the problems have been users not reading the manual and problems with other hardware.