None. You have to cluster several motherboards together. This is an easy way of building a supercomputer for Eng/Sci/Math applications or rendering stuff.
They even cluster PS3s together and the performance is amazing to say the least. http://gravity.phy.umassd.edu/ps3.html http://www.ps3cluster.umassd.edu/
Basically a bunch of computers participating in scientific research. The 'farm' being a 'server farm' which is basically a bunch of PCs you own. http://folding.stanford.edu/
You might want to contribute to the project as it is for a very good cause.
It does not work like that. If you played crysis it would just end up using one card. Forget about desktop applications/games as this is not what this setup is configured to do. It will not run crysis any faster than a single MB with a GTX295.
Essentially what happens is all computations are offloaded to the GPUs (GTX295s) as they are much faster than normal CPUs at performing these types of calculations.