The 5970 is already internally a dual GPU card, where each portion/half has 1 GB of memory (not enough for some of newest DX11 titles at highest 2560x1600 res with enthusiast level AA/AF above 4x)...; the 5970 already supports three monitors.
I've not seen any reviews of dual 5970's, effectively Quad-Crossfire, and am not sure it is even possible...
SOme X58 boards for sure support 3/4 way SLI....; is there even a 3 way X-Fire?
ONce you have so much gpu power that the video is not the bottleneck, then the cpu typically becomes the bottleneck; if you lower the resolution and your framerate increases, then the gpu is still the bottleneck. With three monitors of decent size, depending on AA/AF settings (I'd use no AA/AF if running 3), I'd imagine the gpu is still the bottleneck.