to clarify. you have a gtx580 and you want to go 2 way sli. So you get another gtx580. Standard vram was 1.5GB, but some models shipped with 3GB. Ideally, you want the vram to match(or maybe they need to match. with amd crossfire it just has to be the same chip but can be different models like hd7950 and hd7970 and different amounts of vram). It does not have to be the exact same card or have the exact same clock speeds(for example: a factory overclocked msi twin frozr gtx580 with a reference clocked pny gtx580 would be just fine).