You only need 600 watts for your twin cards....and that's a conservative rating since it comes from the guys offering the warranty
http://www.amd.com/us/products/desktop/graphics/ati-radeon-hd-5000/hd-5870/Pages/ati-radeon-hd-5870-system-requirements.aspx
500 Watt or greater power supply with two 75W 6-pin PCI Express® power connectors recommended (600 Watt and four 6-pin connectors for ATI CrossFireX™ technology in dual mode)
If you are sticking w/ water cooling, use a PSU calculator to figure in the load from the WC system
http://www.antec.outervision.com/
Note the 5970's clock (725-735) is way lower than your original selection's 875 .... an air cooled 5870 is usually 850 ....before you OC it
The 5970 requires a 650 watter....if ya think ya might want to XFire it, an 850 would be in order:
For comparison purposes , the list of 850 watters 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:
Antec SG-850 - 10.0 ($220) **
Antec CP-850 - 10.0 ($110) ** (only fits in Antec 1200, P183, P193 and DF series cases)
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
Here's the list for the 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