www.tomshardware.com/reviews/radeon-hd-7950-review-benchmark,3207.html
Most 7950s hit 1025MHz or higher. The 7870 is close to the 7950 until you OC. I am not sure if the 7870 has much headroom left though since its clocks start so much higher, so the 7950 certainly has more power overall. You'd have to look at some reviews and benchmarks for both overclocked to decide