A good 650W PSU will be plenty for a system with two 6870s. However such a system also needs four PCIe connectors, unfortunately most 650W PSU only come with two. You could use peripheral to PCIe adapters if you are happy to use them.
Assuming you are in the USA and buying from newegg, these are some PSUs which do come with four PCIe connectors:
XFX Pro 650W Core Edition 80Plus Bronze $90 ($10 rebate, $6 shipping)http://www.newegg.com/Product/Product.aspx?Item=N82E16817207014
Xigmatek NRP-PC702 700W 80Plus Bronze $75 ($10 rebate, $2 shipping)http://www.newegg.com/Product/Product.aspx?Item=N82E16817815010