The installation of multiple video cards is a piece of cake under Windows Vista and Windows 7, even if the cards or GPUs are from different manufacturers. Cards from AMD and Nvidia cooperate splendidly in multi-screen scenarios. If you want to increase 3D performance, however, CrossFire and SLI modes always require at least two graphics cards from the same chip manufacturer. Increasing 3D performance by mixing AMD and Nvidia cards is not possible (Ed.: Though this is true in most cases, Lucid's Hydra technology facilitates the mixing and matching of GPUs on boards equipped with its hardware, such as MSI's Big Bang Fuzion).
In order to distribute 3D rendering between two cards in SLI mode, you need to use two identical GPUs and similar frame buffers. Different GPUs do not work, nor do cards based on the same GPU but with different memory capacities, although the current drivers don't actually provide any error messages if cards with different chips are connected via the SLI bridge. Even though the bridge is silently accepted, there is no performance increase, as the driver cannot activate SLI mode. The SLI menu setting will only appear if the installed graphics cards are able to cooperate.
When using two or more Nvidia graphics cards, regardless of whether the GPUs match, one can be dedicated to physics acceleration (PhysX). This can be done even when two cards are working together in an SLI configuration and a third card is added. For these tests, two GeForce GTX 280s and a GeForce GTX 275 are connected with a three-way SLI bridge. The driver recognizes the pair of GeForce GTX 280 cards as an SLI configuration, and offers to reserve one of the GeForce GTX 280 cards or the additional GTX 275 for physics calculations. The drivers have no problem handling the three SLI-bridged graphics cards. The mismatching third card is simply ignored for SLI bonding.
Our benchmarks demonstrate interesting, if not expected, results. Performance is almost doubled by SLI bridging the two GeForce GTX 280 cards, while the additional GeForce GTX 275 has no effect on performance.
After installation, all necessary drivers are installed automatically and assigned to the respective graphics cards. If the installation is fresh, the driver announces an SLI configuration, and all you have to do is to activate it by checking a box. If everything goes well, the configuration then works and the graphics cards divide the 3D rendering workload as soon as you start a game with an SLI-profile assigned to it in Nvidia's driver. The feature can also be activated without using a physical bridge connector between the cards.