To my understanding, SLI has gotten a bad rap because, until recently, the PCIe x16 slots could only accomodate SLI in a x8/x8 configuration, allowing only half the data bandwidth for each card. Due to this, perfomance was barely better than a single card. Newer motherboards have rectified this, allowing both SLI'd cards to run at their full x16/x16 bandwidth.
At Tom's Hardware, they actually recommend 2x Superclocked 512 8800 GT's over the 768 8800 GTX or the superclocked 768 8800 GTX Ultra from a performance vs cost perspective.
Performance would also depend on SLI configuration (the following from Wikipedia... note that in AFR mode NVidea claims 1.9x the performance of a single card... probably due to the master card having to consolidate the output):
Split Frame Rendering (SFR), the first rendering method. This analyzes the rendered image in order to split the workload 50/50 between the two GPUs. To do this, the frame is split horizontally in varying ratios depending on geometry. For example, in a scene where the top half of the frame is mostly empty sky, the dividing line will lower, balancing geometry workload between the two GPUs. This method does not scale geometry or work as well as AFR, however.
Alternate Frame Rendering (AFR), the second rendering method. Here, each GPU renders entire frames in sequence – one GPU processes even frames, and the second processes odd frames, one after the other. When the slave card finishes work on a frame (or part of a frame) the results are sent via the SLI bridge to the master card, which then outputs the completed frames. Ideally, this would result in the rendering time being cut in half, and thus performance from the video cards would double. In their advertising, NVIDIA claims up to 1.9x the performance of one card with the dual-card setup.
SLI Antialiasing. This is a standalone rendering mode that offers up to double the antialiasing performance by splitting the antialiasing workload between the two graphics cards, offering superior image quality. One GPU performs an antialiasing pattern which is slightly offset to the usual pattern (for example, slightly up and to the right), and the second GPU uses a pattern offset by an equal amount in the opposite direction (down and to the left). Compositing both the results gives higher image quality than is normally possible. This mode is not intended for higher frame rates, and can actually lower performance, but is instead intended for games which are not GPU-bound, offering a clearer image in place of better performance. When enabled, SLI Antialiasing offers advanced antialiasing options: SLI 8X, SLI 16X, and SLI 32x (8800-series only). A Quad SLI system is capable of up to SLI 64X antialiasing.
[EDIT] I haven't seen, nor heard of a quad SLI set-up to date although some of the higher end motherboards now allow for tri-SLI with certain 8800 series cards, although space and heat are considerations.
