I am having the same exact problem as Gamble.
I just upgraded my video card to a radeon 4850, so I decided to upgrade to vista x64 at the same time. So I reformatted all my drives, did a clean install of the OS, got the newest driver for the card etc, so I dont think its video card related. Here is a screenshot of the error message, and on the right you can see that only about 30% of my physical ram is used, leaving around 3gigs of physical memory alone left.
The error message changes every time I see it to say I have a different amount, it usually ranges from ~200mb to ~500mb. And this is causing graphical problems in the game where some textures are discolored.