Thanks to Overclock.net, I've learned that any quality 350-400W power supply will easily handle a system that has either a single GT 440 or an 5670 in it. I can try to explain, but I'd rather just post my first PSU recommendation:
400W Antec High Current Gamer for â‚¬55.50
Internally, it's way better than basically any Cooler Master power supply (even I would trust it to power my system).
I tried to find others that are good that don't cost more than this, but the next best one is the Corsair CX430 which is actually not as good as this specific unit I'm recommending. :/