Quad SLI, 3-Way SLI or CrossfireX - what'll it be? PC Games Hardware combines several Radeon HD 4870 X2s as well as Geforce GTX 295 and GTX 285 with Intel's Core i7 and checks which configuration is best.
Quad Crossfire versus Quad SLI: Introduction
3-Way SLI reviewed [Source: view picture gallery]
In times of big displays, a trend for HD gaming and games like Crysis, a single graphics card reaches its limits quite fast. AMD and Nvidia are of the fact and developed technologies that are supposed to increase the framerates drastically: Scalable Link Interface (SLI) and Crossfire. Initially intended for two cards respectively GPUs only, today combination of up to four GPUs are possible. Nvidia talks about Quad and 3-Way SLI and AMD uses the term CrossfireX. For this review we pushed the limits and test the best available hardware combinations. Quad Crossfire versus Quad SLI: Survey and Configuration
"True” SLI or Crossfire is not working on every motherboard. Two PCI-Express slots - 16 PCI-E 2.0 lanes would be ideal - are required at least and only Intel's X58 supports both technologies currently. Apart from that you need an Nforce board for SLI and motherboard with an Intel or AMD chipset for Crossfire. The cards are communicating via SLI/CF bridges or via the PCI-E slot on weaker boards.
In principle you can combine Geforce cards of the same type only, which don't necessarily have to be from the same manufacturer. If one of the cards is overclocked, the driver tries to balance this - so OC should be avoided. Different amounts of video memory of the linked cards can also result in problems. High-end cards like the GTX 285 have two SLI connectors and thus only such cards are 3-Way SLI compatible. Smaller cards can be used as dedicated Physx accelerators without an SLI bridge. See Mirror's Edge: Physx effects benchmark review
for more information.
The same applies for AMD cards in matters of OC and video memory, but there are more combinations. At game.amd.com you can find a list of all possible combinations. You can, for example, link all cards of the HD 4800 series, but practically this should be avoided because of the named reasons. Furthermore a 4870 X2 only communicates with a 4830 via the slower PCI-E slot only. As it is the case with Nvidia, too, you can only link up to three separate cards.