Until recently it was working fine, but I started experiencing major slowdowns. When I reboot, the initial memory test reports that I have 4864MB of RAM, 768MB more than I actually have. It's a strange pattern though, because it counts normally up until 3328MB, 768MB less, pauses, then skips straight up to 4096MB and continues to count for another 768MB. It's very strange indeed.
I've tried testing both chips individually, both report the correct 2048MB and both run fine individually in either slot, but when I put both in, no go. I bought another 2GB chip almost exactly the same model, and no matter what combination, it still thinks there's an extra 768MB there... The OS (WIN7 x64) runs extremely slow and unstable when I have both chips in, but runs otherwise fine with only one.
A bit of a long shot here but it may be reporting your GPU memory as physical memory as well. Do you have a GPU with 768 MB of RAM? I am at work so I cannot do too much digging but I am just trying to wrap my head around this one.