Re: GTX 295 (dual core) not working with two screens and compositing?
For what it's worth, I have a GTX 295 working with twinview, with the second GPU disabled. In fact, this is how X configured itself out of the box - without the 'MultiGPU' option in Xorg.conf, the second core just sits idle and GPU-0 drives both screens. Unless you're planning on playing high end games, a single core of a GTX295 should do just about anything to do with the desktop/compositing just as well for two monitors. The most intensive thing I do graphically in linux is play WoW, and that manages 60FPS on one monitor (with compositing enabled, no less) without the second core.
That said, the multi-monitor support in linux is pretty depressing - only proprietary driver hacks or a slow poorly supported extension... srsly?