HardwareUpgrade just posted an in-depth analysis of the GeForce 8800 GTX and GTS while under SLI. Here's a taster:

For consumers who have a budget with quite a large amount of headroom, two GeForce 8800 GTX video cards combined using SLI technology makes the fastest gaming solution on the market. Two of these video cards, though, don't come very cheap. The average price for a single card is around $650 so two of these cards weigh in at around $1300 USD. For our UK readers, this is around 665.91 pounds. Availability for 8800 GTX video cards, however, are still low and so it might prove difficult getting one card let alone two.

Scalability for the 8800 series going from one video card to two was impressive. The GeForce 8000 series currently have excellent DirectX 9 performance and as their architectures are designed for DirectX 10 they will be able to take immediate advantage of DirectX 10 games when they are released. Here is a table that summarizes the scalability of the tested video cards.

For the price of a GeForce 8800GTX SLI system, it is important to realize that $1300 USD will not be your final cost. If you are constructing a system from scratch, you would still have to factor in the price of the motherboard and, in order to not limit GPU performance with the CPU, factor in the price of a high-end CPU. You would also have to purchase an adequate power supply to power this system. It wouldn't make sense to spend so much money on a powerful system just to run games at low resolutions, and so we naturally assume you will be buying a display that supports extremely high resolutions. For our testing, we used a Dell 3007WFP, a flat panel LCD monitor which runs in the price range of around $1,000. Add up the price of all of these components, and the final cost comes out to be quite hefty.