Re: 2x7800gtx driving 2 LCD's.
You don't need the SLI bridge for running separate displays, but you probably do want to set the motherboard dongle to multi-card mode, as I think that affects the PCI-E lane width. You can check the lane width of each GPU in nvidia-settings after X is running. As for running multiple X servers (so-called "multiseat"), such configurations are not officially supported, but may work. To configure a normal multi-screen X server, run nvidia-xconfig -a