A system with a single HD 5850 requires a 500W PSU with at least 32A on the 12V rail and two 75W 6-pin PCIe power connectors. At full load, this system will consume around 350W.
The Corsair CX500 has 34A on the 12V rails and two 6+2pin PCIe power connectors and is sufficient to power your system. If you plan on OCing the CPU/GPU then I would recommend a PSU with more stout 12V rail(s); something with 40A+ on the 12V rails like the Seasonic M12II-520, Antec HCG 520, or the XFX Core 550W.
There is a lot of debate back and forth, but it doesn't really matter for the average user. I have a Seasonic M12II-520 running an HD 6950 with absolutely no issues; the XFX 550W is made by Seasonic and is also a great little PSU. Bottom line...get either one and be happy with a great PSU.