I agree, the nautilus is a decent CPU cooler and will handle some good overclocking, but shouldn't be considered for more than that. As far as MB cooling, the heatpipe setup on the A8N32 is one of the better cooling setups, plus I doubt you'll find any waterblocks that will fit while your video cards are in place. Just make sure you get the board with the optional fan which was made to use when the CPU is watercooled.
The 7800GTs have a software voltage mod that lets you get over 550, but you'll need to keep them cool..dangerden has a real nice SLI cooling setup...
www.frozencpu.com usually has them in stock. They also carry everything you'll need for a custom WC setup.