Can i use 2 different models of ati for crossfire?

Cant do it, you dont have to crossfire the same exact model but you do have limited options on what will work together, see the chart below.

i am not getting this chart help plz

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.

ohh yeah i got it thanks alot friends.