Hey Tigger,
dont switch unless you can sell your current card. The 5830 can serve you very well for a while, i wouldnt throw 120 bucks out for 15% improvement (considering the 5850 can be overclocked too).
One more thing: hybrid crossfire. I dont know how it works, never tryed it. But if you make sure it works, also your motherboard/PSU supports it... i would try. A 5830+ a 5850 together should beat a 6970. 2500 stream processors, like 100 texturers, 48 ROPs... a good base for a triple head setup. Consider this, do some research and make your decision.
Best wishes