Iria

"This card was the best overclockable card I have tested out of the three HD 4870 cards I have reviewed. The limits allowable in the Catalyst Control Center are 790MHz on the core and 1100MHz on the memory. I was almost able to max them out with the Sapphire HD 4870 reaching 785MHz on the GPU and 1095MHz on the memory, for an effective speed of 4380MHz. That is a very fast memory speed. Anything higher and the screen would artifact or I would get a BSOD, so the overclocked tests will be run at 785/1095MHz."
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Source: OCC
Check out our review here