For gamers, 2 GTX 295s is the best - drivers don't support more than 4 GPUs working together for a single monitor. 8GPUs (4 GTX 295s) is only useful for driving a whole bunch of monitors or for something like Folding@home.
maybe someone has an amplifier in their pc to run their home theater while using a super overclocked i7 using a phase change cooler and 4X GTX 295s to run folding@home at the same time.
SLi only allows for 2 GTX295, so for gaming thats the max setup.
However, so long as you have 4 PCI-E slots, theres nothing stopping you from loading up a mobo with 4 GTX295 for the purpose of GPGPU applications such as folding.
why would you want 4x295gtx graphics cards? for gamers who play on 30" monitors?
Some people , like myself use GPU's for other task's not just gaming. I don't use GPU's for gaming at all as a matter of fact. I use it to password crack or decrpyt password hashs. Using cuda cores on GPU's out preforms the power of CPU's any day of the week. I hope this answers your question. A good exsample if you are interested in reading up on this would be here at this site. http://mytechencounters.wordpress.com/2011/04/03/gpu-password-cracking-crack-a-windows-password-using-a-graphic-card/
By using CUDA compatible GPU's , I can speed up a process that would normally take YEARS, into hours or a few days. hope this was of some help from a non-gamers view. -814ck0u7