That message would have nothing to do with your video card or VRAM. That would be a physical RAM error message, so there's no point in looking at your graphics mods for a solution. Something else is wrong as graphics have very little impact on your physical RAM, only on your VRAM. It could be an indication of a hardware problem as it's very rare to be running out of memory playing Oblivion, even with a heavily modded setup.
"very rare to be running out of memory playing Oblivion" <-- memory = VRAM here
Edit: 2-3 hours is a more normal limit (largely set by Oblivion hitting its loaned, not VRAM, memory limit.)