Something is wrong. Run memtest86+ which uses it's own OS. It should identify all the ram and test it out.
Sometimes the motherboard requires a bios update to fix some ram issues. Do not update the bios unless it will fix a problem you have. Also, is the ram installed in the proper slots? I would hope that Alienware would know how to do this properly, but It might cause the symptoms you are seeing. Download the motherboard manual and verify which slots should be used with only two sticks.
CPU-Z will identify the mobo and the ram.