You probably could fix this situation if you could raise your ram voltage or adjust the ram timings, sadly you can't. It is possible that 3d graphics uncover or create a error at some ram address that was previously (lucky for you) never encountered before.
To prove all this out, you could d/l memtest at memtest.org, it creates a dos boot disk and run it. This runs without windows obviously. I'd almost surely bet that it will not show errors without the 2~512's , but will with.