If I have a 6990 4GB card in primary, and I crossfire a 6970 2GB, does this reduce the amount of available memory for card 1? I've read both that it does and does not, because each card uses its own memory.
Either way, the reviews on 6990 + 6970 in 3 monitor gaming (eyefinity) certainly shows that its quite a bit better with the 6970 added in.
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You will have a default 2Gb per GPU same as before with the 6990 2 x GPU with 2GB each.