http://www.amd.com/us/products/desktop/graphics/ati-radeon-hd-5000/hd-5770/Pages/ati-radeon-hd-5770-system-requirements.aspx
450 Watt or greater power supply with one 75W 6-pin PCI Express® power connectors recommended (600 Watt and two 6-pin connectors for ATI CrossFireX™ technology in dual mode)
The amp requirement fir the 5770 is often overstated.
If you aren't overclocking, then the above will work.....if doing serious OC's, the sweet spot will be 650 watts for a single GPU setup and 850 for twin GFX cards....the latter also leaves room for a dedicated PhysX card. The reason I says weet spot is that quite often a PSU of 500 watts will be almost the same price as the 650 watter. For example:
500 watts $69
http://www.newegg.com/Product/Product.aspx?Item=N82E16817371007
650 watts $79
http://www.newegg.com/Product/Product.aspx?Item=N82E16817371015
For comparison purposes , the list below contains performance ratings (10 scale) at jonnyguru.com / prices as per newegg on 02/21/2010 and ones w/ double asterisks (**) made it onto silentpcreview.com's Editor's Choice List meaning they excelled at both electronic and acoustic performance:
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
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
NFS - Not For Sale / NLA = No Longer Available / OOS - Out of Stock