SLI/Crossfire and other Multigpu configurations
3dfx: where it all started
SLI, originally introduced by 3dfx with their Voodoo2 cards, stood for scan line interleave and it was just that: two video processsors would work towards rendering a frame: one GPU would process the odd lines and the other would render the even ones. 3dfx's implemented SLI using two cards connected by a dongle (which suffers from timing issues since the scene reconstruction occured after the video had been sent to the RAMDACs) as well as having two GPUs on the same board. For the most part, when 3dfx dissapeared, SLI somewhat dissappeared.
nVidia
More recently, nVidia has ressurected the idea of multi-GPU rendering (they weren't the only bunch to do so, just the only one really successful at it). As things currently stand, SLI stands for scalable link interface. The principle here is the same: take to identical and compatible video cards (that is, same make and model), link them up and have the two cards jointly render a scene. There is a theoretical performance increase of 100% however it will never hit that mark due to load-balancing overhead).
A few intrepid manufacturers like Gigabyte and ASUS have gone a step further and put two GPUS and two sets of memory together on single card (i.e., single card SLI). Also, with the latest driver revisions (ok, for some time now), SLI no longer requires that the cards be exactly identical: in fact, the individual cards can be run asynchronously of each other (also, the PEG link is no longer required per se however not using it comes at a roughly 5% performance cost).
ATi's competing technology, Crossfire is essentially the same principle however ATi's crossfire is significantly more 'open'. Whereas nVidia's SLI requires an NF4SLI chipset to be paired with two identical cards for SLI to be enabled, ATI's solution works across multiple chipsets (RX200 and i955X) and two identical cards are not required. In short, Crossfire is slightly more flexible and lenient as far as product selection goes but at the end of the day, as far as the consumer is concerned, it's essentially the same deal. As noted above, SLI has become more flexible as of late however its interesting to note that Crossfire was developed with the flexibility in mind.
