realhardtech (this is their expertise) suggests 575w for a sys w/2x7850,
if you have a non-reference(common) you will need 4 pci-e connectors
vs 2 for a reference version. I'll assume you need 4. You can post the make/model
of your 7850, if you need only 2 pci-e connectors, a lot more psu options.
Also, you can use molex to pci-e adpaters as well.