The various systems
To test the different multi-card solutions, we should speak of systems because not all are compatible with all motherboards. Thus, Nvidia’s SLI is exclusively reserved to its own chipsets (except with Intel’s Skulltrail platform). As for triple-SLI, it’s currently limited to the nForce 680i and 780i. On AMD’s side, there is more flexibility because CrossFire is functional with, in addition to its own chipsets, the following ones from Intel : P35, P965, 975X and X38.
Our CrossFire platform was based on the X38 which we preferred to an AMD chipset because the Phenom platform isn’t too convincing yet. Making the transition to a Core 2 and still keeping CrossFire, an Intel chipset had to be used. We opted for an Asus P5E motherboard which allowed us to stay in DDR2 like with Nvidia platforms. Two Radeon HD 3870s were combined for graphics.
For the SLI platform, we used an eVGA nForce 680i motherboard. It is limited to PCI Express 1.1 but as you can see in our
review of the nForce 780i, its PCI Express 2.0 support isn’t ideal and doesn’t add any significant gains. Also, the GeForce 8800 Ultra does not support PCI Express 2.0. In addition to the GeForce 8800 Ultras, we also tested a couple of GeForce 8800 GTs.
Finally, the 3-way SLI platform is of course identical except there are 3 GeForce 8800 Ultras combined to create the ultimate system in terms of current graphic power.
Specifications
While 3-way SLI allows blowing away graphic capabilities, it doesn’t come without a price as you will have to invest in three GeForce 8800 Ultras or GTXs.
You may recall that while calculation power and memory bandwidth are added up in a multi-GPU system, it isn’t the same for the quantity of memory as data is duplicated in each local memory. Also, multi-GPU functioning means use of more memory because a zone on the "master" graphic card has to be set aside whose sole purpose is to store the work of other GPUs. Consequently, the pressure on memory is increased in both SLI and CrossFire. You should therefore try and avoid at all costs combining low memory graphic cards (HD 3850 256 MB and other GeForce 8800 GT 256 MB), even though they may appear to be good deals.
