I would not suggest it. When one GPU is WAY slower than the other, it creates problems. When one GPU is a different architecture than the other, it creates problems. When two GPUs are connected, it creates problems.
Only link cards that are nearly the same, and even then you'll run into problems.
And really, micro stuttering would probably be horrid with that lop-sided set up... Your GPU is already within top five single GPUs... If you want more performance, look to OC your CPU or get faster RAM.
If I were you, I'd sell the 5870 and whatever else to get an 8-core AMD or i7 unlocked CPU and do that 7970 some justice.