SLI only does 1 monitor.. you would have to disable SLI to drive a second. Hence, needing a 3rd card to have SLI enabled to run a 2nd monitor. I think you might need a small PCI graphics card to run the monitor since the 8800's take up so much room depending on your motherboard layout.
something Nvidia (doesn't have to be Nvidia, but might make driver issues easier), cool running, no power cable needed, if you have a pci-e slot for it, maybe the cheapest 6600 or maybe 6200... if you want to use a reg pci slot, go eBay or compgeeks for older cards..
What are you wanting to do with the second monitor. If it's nothing insane (like rendering while gaming, or watching an HD movie while rendering a 4,000hr 3D image) then just get a old PCI Nvidia Geforce 2 MX-400 and throw in there. It'll do everything you'd need to display IM windows, mediaplaers, web pages, etc.
Ya, if you aren't doing anything intense (like you just want the 2nd monitor for desktop work) I'd opt for something low power mainly because with 2x GTXs you are talking some serious heat and power consumption. No need to make it worse if there's not a reason. If you are thinking of gaming, I'd say maybe you are better off with 2 GTXs not SLI'd, one driving each monitor. By all accounts the 8800 GTX is fast enough to rip apart any game even on a large monitor by itself, so SLI is probably largely unnecessary at this point. You could always SLI them later, should you find it to be necessary.
I am going to be driving a dell 30" monitor with the two 8800 GTX at max resolution for gaming. This second monitor is just for browsing the internet or running simple apps.
My goal is to get a third video card to fit inside the middle PCIe slot. I'm concerned about spacing between the two 8800's because of the size of the video cards.
If anyone has this working now I'm curious what video card they are using to do this.
PCI video card is not an option as there are only two PCI slots on the i680 motherboards. One is completely blocked by the 8800gtx card and the other will be for a PCI soundcard.
Grab something like a 7300 then. They are small, don't need much power (no external power connector) and will do find for 2d work, and even light 3d. Should do the trick.
Just disable TurboCache if you can since you won't need to use the card for 3D applications. I'm sure it'll produce less heat than a 7300 series, but if it doesn't, then it's definately cheaper.