You should try and match your cards up identically. That said, any 7970 will work with any 7970.
Here's the issue though. The clock speeds need to be the same. If you buy the factory OC version, it'll just be downclocked to match the speed of the other one. Therefore, grab the smae one you have now, without a doubt.
I've been thinking about this as well. Since i can't find the original MSI R7970 for sale the only one i'm seeing now is the MSI OC 7970. But why couldn't i just overclock my original card instead of downclocking the new card?