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Can I mix and match graphics cards with different sizes of memory?
No. For example, an XXXGT 512MB cannot be paired with a XXXGT 1GB in an SLI configuration.
For example, it is rather difficult to get two Nvidia cards obtained from different vendors, which deviate from the reference specification, to cooperate in SLI. From a pool of six different GeForce GTX 580 cards, only two would work together consistently. From a pool of five different GeForce GTX 570s, only three would. We had problems mixing clock rates, BIOSes, supporting platforms, and so on.
I called evga with that same question and here's the answer i got: You can not SLI regular 580 with the 3GB version!
For example, it is rather difficult to get two Nvidia cards obtained from different vendors, which deviate from the reference specification, to cooperate in SLI. From a pool of six different GeForce GTX 580 cards, only two would work together consistently. From a pool of five different GeForce GTX 570s, only three would. We had problems mixing clock rates, BIOSes, supporting platforms, and so on.
Even two 1.5GB GTX 580's don't always play nicely in SLI.