If i wanted to run a single ATI video card i know a 650w is plenty what power supply would i need if i wanna run 2 ATI video cards.. Mind there is a ASUS Crosshair IV Formula AM3 board, and a AMD Phenom II X6 1090T Black Edition Thuban 3.2GHz.. do i need more power?
Aww Maziar, you're no fun today; I was going to suggest that a 300W PSU would have no trouble with a pair of HD5450s...
Anyway, for a pair of HD5850s, the operative word here along with the 650W was "good." Quality modern PSUs have full range active PFC (no little voltage switch) and 80+ certification. Antec, Seasonic, Corsair, Enermax, and XFX are among the better brands. Other than Enermax, avoid any PSU with "max" in its name, as it is most likely a reference to the size of the pyrotechnic display or the number of parts it will take with it when it dies.
what are your thoughts on a XFX HD585XZABC Radeon HD 5850 Black Edition Video Card - 1GB GDDR5, PCI Express 2.0
as JTT and Maziar stated, a 650w from seasonic, corsair, xfx, antec, silverston, enermax will do the job easily. the corsair Corsair HX 650W would be my personal choice.
the 875w is overkill, as stated above.