Q: What cards can I use together in SLI? What are the requirements?
A: Right now, users are limited to using two (or more, if applicable) cards that are from the same series (7600, 7950, 8800, etc.) and are the same model (GS, GT, GTX, and so on.) However, the manufacturer, VGA BIOS, and clock speeds may all differ. When using two differently-clocked cards together, it's recommended that the slower card be installed in the primary PCI-E slot. This will automatically gear down the other card's clock speeds to match those of the master card, which will keep them in sync. Depending on the driver support the faster card may be installed as the master, which will raise the clock speeds of the other card, but this may not always be the case. Installing the faster card as the primary GPU can still sync the cards to the lower speeds or the overclock may not be forced on the other card and render the setup incompatible in that configuration.
Q: My cards are the same, but have different video memory sizes. Can I use them in SLI?
A: First and foremost, this is an unofficial workaround. To use SLI with cards that have mixed VRAM sizes, install the video driver as per usual. Then download and install the Coolbits registry string, then modify its value through the registry editor (Run > "regedit") and change it to 18. Once Coolbits has been changed, search the registry for "MB_SkipFBSizeCompare." This is the crucial part of the process, because if that string hasn't been created, the workaround wasn't successful. This may be due to a recent behavior current drivers are exhibiting that prevent that string from being made, so if the string doesn't show up, simply uninstall the video driver and roll back to an older version. For simplicity's sake, it's best that users use two identical cards for SLI.
