I have a Toshiba A665-S6065 which I bought about 1.5 months ago. It's been working perfectly with windows 7 (64 bit) and ubuntu (64 bit) dual booting.
However, a few days ago I noticed that only 1940 mb of ram were being used by linux. I know this definitely was not the case several weeks ago.
I tried rebooting into windows 7 to see if it was a problem with linux, but again, only half my ram was actually being used by windows. Looking at the bios settings page, it says that the computer has 4gb of ram. I ran memtest86, and all 4gb were recognized and no errors were found.
I have no idea what's causing this problem, and if I can't fix it, I'm going to just return it for another laptop.