Re: PSU for 5850
The computer may not start, which would be the best thing. Or you could get it all up and running, and literally fry the PSU or Video cards. If it does start up, get a voltage monitor! Software programs should do just fine. If/When it's booted up, check the 12V voltage.
I took a chance and tried my luck with powering a 4870 with an Antec BP-350. I was lucky as it worked, and kept good voltage, as long as I didn't overclock the video card.
ASUS M5A99FX Pro R2.0 : AMD FX 9370 : Corsair H100i GTX Cooler : 16GB Corsair Vengeance Blue 1600 (4x4Gb) : XFX 7970 Ghz 3GB x 2 : PNY XLR8 256GB SSD : Seagate 1TB : Antec True Power Quattro 1000W : Thermaltake G41