First question regarding the 8800 GT, can I have 2 cards from different manufacturers in SLi mode? Second question, if I had two 9800 GT cards, but they had different amounts of memory but same clock speeds will that difference in memory affect SLi mode?
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question 1 - yes you can have two different manufacturers
question 2 - the two cards would default to the lowest common specs.
1) yes 2) you would effectively only get the smaller memory (512MB+1GB would act like 512MB card with roughly double power since each card needs its own memory)