Ok. I had the exact same problem. I ran a memory tester, everything checked out. So I physically took the RAM out, and tested each individual stick. Turns out one of my sticks of RAM is faulty. I don't know why the checker didn't catch it, or why Windows saw it but couldn't use it. But I do know I replaced the Ram with another stick, and now I have 4GB of Usable Ram.
The RAM I had was part number OCZ3G1600LV4GK