Value & ConclusionNVIDIA GeForce GTX 560 Ti SLI is a compelling multi-GPU solution. It supercharges the single GTX 560 Ti, making it an enthusiast-grade solution that can run any game at any resolution with lots of eye-candy. It outperforms NVIDIA's top high-end GPU, the GTX 580, and edges past the Radeon HD 5970, which is still the single fastest graphics card. GTX 560 Ti SLI gives you a richer feature-set, including 3D Vision Surround, which lets you span a single display-head over multiple physical displays. You'd need two GPUs to make a decently-detailed game playable that that high resolution anyway, so needing two graphics card to use that feature is kind of warranted.
At close to $500, the NVIDIA GeForce GTX 560 Ti is a decent multi-GPU upgrade option. Notice I said upgrade option, and not something you'd want to buy outright. The GeForce GTX 560 Ti lacks 3-way or 4-way SLI support, and hence it's suited more for people who buy one card and in the future add another one. If you have $500 to spend right away, you're better off buying a GeForce GTX 580, and leaving room for three more cards, provided your PSU and motherboard are cool with it.