I had this problem with an embedded nvidia GPU on the motherboard. (A GeForce 6150SE component as part of the nForce 430 chipset).
Opening nvidia-settings, I noted that the driver was convinced that the card had 256MB of RAM even though the RAM was shared system RAM, and the BIOS had it set to 64MB.
Setting the BIOS allocation of RAM to 256MB rectified the problem with white windows, as in, recipes that previously reliably produced white windows no longer did so.
However, the graphics performance is now a little sluggish, with operations like dragging windows and switching workspaces eliciting pauses.
While your problem appears to be with Natty, this was on Oneiric.