Ok so I just crossfired a XFX ATI HD4850 1GB and a XFX ATI HD4850 512MB and when I go to Catalyst Control Center, under Crossfire it has the the enable crossfire checked. but it tells me "you are running two different GPU's but they can still be crossfired" (or somthing along those lines). Is it saying this cause I have one 1GB and another 512MB? and will it still perform the same as if I had 2 512MB cards?
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Yes that is why it is telling you that.
If you have one 512mb and one 1gb card in Crossfire, it will drop the 1gb card down to 512mb so they can work together. You have to have two cards with the same memory size to correctly run Crossfire.
You should probably sell the 512mb and get the 1gb instead... You are wasting the performance of the 1gb card. ( Not that theres much difference on that type of card )