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Did you read the manual about crossfire? Which motherboard do you have?
"To enable dual mode ATI CrossFireX™ technology, a second ATI Radeon™ HD 4600 Series graphics card, an ATI CrossFireX Ready motherboard is required. Enable Software Crossfire in ATI Catalyst™."
Did you already own an HD4670 & add another later or did you deliberately buy two of them? A single HD 4850 (about $110) beats 2 x HD4670.