A system with a single HD 7850 requires a 450W PSU with one 75W 6-pin PCIe power connector and should have 28A on the 12V rails. A system with two HD 7850s running in two-way Crossfire mode requires a 600W PSU with two 75W 6-pin PCIe power connectors and should have at least 36A on the 12V rails. The HX650 has 52A or 54A on the 12V rail (new version has 54A and the original has 52A) and both have sufficient PCIe power connectors. Yes, the HX650 is sufficient for running two HD7850s in CF.
If OP gets HD7850s that require two 6-pin PCIe power connectors then the new HX650 does not have sufficient PCIe connectors. The new 80+ Gold HX650, model HX650 (75-001230), only has two 75W 6-pin PCIe power connectors.
The older HX650, model CMPSU-650HX, had four 75W 6-pin PCIe power connectors and would support a Crossfire of HD7850s requiring two PCie connectors each for a total of four.
Bottom line is that there are differences between the old and new HX650 models; be sure which model you are getting and how many PCIe power connectors each HD7850 requires.