Have one question.

I was planing to try out some crossfire, so i bought a new powercolor 3870 from ebay. Today i recieved it and i see that i got an DDR3 version, and my other powercolor 3870 is DDR4 that i bought earlier this year.
What should i do ? Can i run these together or will it fail.

DDR4: 1125mhz on memorys
DDR3: 900mhz on memorys

Please help me out here.