Geraintl, your Coolermaster is not even close to being maxxed out. Reviews show SLI 970 systems to have a max power draw in the region of only 450w so you have 300w of spare headroom on your 744w 12v rail.
Jonmdj, a decent quality 550-650w psu would be more than enough for a single card. However, you say you want room to upgrade in the future so does that mean a additional 390x? If so you want a 850w+ psu. Here's some suggestions:-
550-650w
Non-Modular (fixed cables)
My basket at Overclockers UK:
The Antec units are built by Seasonic and both have a single 12v rail with the 550w having a 540w 12v rail and the 650w having a 648w 12v rail. Both have a 5 year warranty.
The Superflowers are also a single 12v rail design and the 550w version has 546w on it's 12v rail while the 650w version has 648w on it's 12v rail. Both units also have a 5 year warranty.
Modular (removeable cables)
My basket at Overclockers UK:
The Superflower units are both single 12v rail designs with the 550w being able to deliver the whole of it's 550w on the 12v rail while the 650w version can deliver 649w on it's 12v rail. Both have 5 year warranties. Be warned that both of these units have bright white leds where you plug the cables in and they cannot be turned off.
The EVGA units are built by Seasonic and once again are both single 12v rail units. The 550w version has a 540w 12v rail while the 650w version has a 648w 12v rail. Both have a 5 year warranty.
The XFX units are also Seasonic built and are both single 12v rail designs. The 550w version has a 540w 12v rail while the 650w version has a 648w 12v rail. Both units once again have a 5 year warranty.
EVGA G2 650w - Basically a Superflower Leadex gold with a different fan, no leds and a longer warranty. Single 650w 12v rail, fully modular and a 7 year warranty.
Other units worth checking out but not available from OCUK due to being out of stock or simply not sold here are:-
EVGA G2 550w - Basically a Superflower Leadex gold with a different fan, no leds and a longer warranty. Single 550w 12v rail, fully modular and a 7 year warranty.
EVGA GQ 650w - FSP built semi-modular unit with a single 12v rail of 650w and a 5 year warranty.
If it were me I would be tracking down a EVGA G2 as they are cracking units and argueably one of the best platforms currently around.
850w+
My basket at Overclockers UK:
The XFX XTR 850w and 1050w are both Seasonic built units with the 850w having a single 840w 12v rail and the 1050w unit having a single 1044w 12v rail. Both have a 5 year warranty.
The pair of Superflower 850w units both have a single 12v rail of 850w each and both have a 5 year warranty. Be warned though that both have bright white leds where you plug the cables in and they cannot be turned off.
The EVGA Supernova G2 850w is basically a Superflower Leadex Gold with a different fan, no leds and a much longer warranty of 10 years. As with the Superflower's, it has a single 850w 12v rail.
The Corsair RM850X is a CWT built unit with a single 12v rail of 850w and has a 10 year warranty.
