The short explanation is you can only crossfire cards with the identical first 2 digits. Eg. HD4650 with HD4670 and HD5770 with HD5750.
(there are exceptions with this rule however, like how the HD5970 can crossfire with any HD58XX)
It's like a map. On the left it Card 1, while the top is Card 2. You have a 5770, so look for that on the left, and follow the squares to the right. Whatever is a red square, just go above the square and look at what card it falls under. You can CF with a 5770, or a 5750.