What is wrong with the 6870's? The hardware mining comparison page shows they are pretty much equal to 5830's for mining. around 300mh/s. I will have a couple coming online tonight. Paid around $150 each (after rebate and selling the Shogun game that comes with them on ebay).
I could be wrong, but I believe the 6870's consume more power than the 5830's. So they may preform about the same, but you'll pay more in electricity for the 6870's than the 5830's if that's the case...