Well, it seems they still haven't made any progress at making the Compiz graphics effects work on a multi-GPU setup, but you can run two X screens each using two monitors and keep the Compiz effects. If you want to grab and drag windows across all four monitors, you lose the cool graphics effects.

Here's what it might look like though using four monitors of three different sizes using xinerama without Compiz on Ubuntu Lucid 10.04 with two nVidia cards, each driving two monitors: