Here's the review on the BFG....gets a 7 outta 10.
http://www.jonnyguru.com/modules.php?name=NDReviews&op=Story5&reid=151
For a 5850, a 500 watter will do just fine....The EA-500 for example gets a 9.5 performance rating
http://www.jonnyguru.com/modules.php?name=NDReviews&op=Story&reid=35
To my mind however, the sweet spots are at 650 (single GFX and OC'ing) and 850 watts (twin GFX and OC'ing). The price difference between an 500 watter and 650 are small and PSU's hit their peak efficiency at about 50% load.
For comparison purposes , the list below contains performance ratings (10 scale) at jonnyguru.com / prices as per newegg on 02/26/2010 and ones w/ double asteris (**) made it onto silentpcreview.com's Editor's Choice List:
Note: Just because you see a brand name on this list, never make the assumption that their other lines are of matching quality. Silverstone for example has other models w/ 6.0 and 7.0 ratings.
650 watters
Antec Signature 650W (10.0) OOS **
Seasonic X-650 650W (10.0) $160 **
Silverstone Olympia OP650 (10.0) NFS
Thermaltake Toughpower QFan 650W (9.5) NLA
Antec Earthwatts EA650 650W (9.0) $75
Antec True Power Trio 650W (7.5) NLA
XFX XPS 650W XXX Edition Power Supply (7.0) NFS
Antec NeoPower Blue 650W (6.5) NLA
BFG BFGR650PSU 650W (6.0) NLA
NorthQ Black Magic Flex 650W PSU (5.0) NFS
850 watters
Antec SG-850 - 10.0 ($220) **
Antec CP-850 - 10.0 ($110) ** (requires compatible case)
XFX 850W Black Edition 10.0 ($185)
Corsair HX850 850W - 10.0 ($180)
Antec TruePower Quattro (TPQ-850) - 9.5 ($150)
Corsair TX850 - 9.5 ($140)
Seasonic M12D 850W - 9.5 (NLA) **
NorthQ Giant Connector 850W - 9.5 (NFS)
Etasis ET850 - 9.5 (NLA)
Silverstone Zeus ST85ZF - 9.5 (NLA)
Enermax Revolution 85+ 850W - 9.0 ($250)
Thrmaltake Toughpower XT 850W - 8.5 ($216)
OCZ Z Series 850W - 8.5 ($200)
NorthQ Black Magic Flex 850W - 8.0 (NFS)
Coolmax CTG-850 - 8.0 (NFS)
SilverStone Decathlon DA850 - 7.5 ($230)
Enermax Galaxy DXX 850W EGX850EWL - 7.5 (NFS)
SilverStone Strider ST85F - 7.0
Silverstone Element ST85EF 850W - 6.0
NFS - Not For Sale / NLA = No Longer Available