There both a Radeon HD 5770 (Juniper XT), but the second card is the SAPPHIRE Vapor-X and is PCI Express 2.1 while my current 5770 is PCI Express 2.0. My MOBO has 2 x PCI-E 2.0 x16 slots (@x16 or @x8, x8).
Will these 2 cards work together without problems?
Current build has plenty of power with a 1k PS, my MOBO is a GA-P55-UD4P and my OS is Win 7 Ultimate 64bit.
Basically you can crossfire any 57xx card with any other 57xx card so that means you can crossfire a 5770 with a 5750. What will happen is that when crossfired, the higher card(5770) will dumb itself down to the level of the lower card(5750) so it can scale at the same level.
When using 2 x 5770's you needn't worry about this.