Sell the 4890 to somebody...it is a legacy/non-supported DX10 card. Match the 7770 with what it is able to crossfire to...and try to get the same brand/model. The question I have is, why would you want to crossfire and have all the problems that causes? All of this is moot if you don't have a OS , PS , and processor that can handle two cards.
Intel I7 960 @ 3.87ghz *Intel DX58SO *HIS HD6970 2gb *Corsair TX650M *2x4gb Corsair XMS3 *WD Black 1TB *Windows 7 Home Premium 64bit *Gateway HD2401 24" 2ms 1920x1200 ***Asus N71Jq laptop *Intel I7 720QM Processor *Mobility Radeon HD5730 1gb *8gb Ram *Windows 7 64bit
* A clear conscience is usually a sign of bad memory *