Tady pise neco jineho:

All GPUs must have the same amount of VRAM and an identical Bus Width
-- You cannot use a "2GB" version with a "4GB" version or "3GB" version with a "6GB" version in SLI or a 1GB 192 bit bus with a 1GB 256 bit bus Etc.
Note: It used to be possible to enable SLI on cards with differing VRAM but this is no longer available.
Note: In SLI the amount of usable VRAM does not stack. It works like a RAID1 hard disk array where data is mirrored between each GPU. For example, having 3 GPUs with 4GB VRAM means you will only have 4GB available, not 12GB