Do you plan on crossfiring?
If not, a good 500-600W PSU will do it.
if looking at crossfire, aim nearer 750W.
Look at known, respected brands, such as Corsair, (some) Seasonic, PC Power and Cooling, etc. check decent PSU reviews on bit-tech or jonnyguru and feel free to come back and ask when you think you've chosen one.
Not planning on cross firing atm, as the 5750 should be right for a fair while you'd think. When the time comes where it is starting to come out of date I will just replace the card I guess. This is for my brothers computer, atm I have a TX750 Corsair in mine, probably overkill for what I have but yes, it is good to at least have the option of Xfire.. hmm. Will have to look around and see the differences in prices now
PCI Express® based PC is required with one X16 lane graphics slot available on the motherboard 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)
Certified power supplies are recommended. Refer to http://ati.amd.com/certifiedPSU for a list of Certified products
Minimum 1GB of system memory
Installation software requires CD-ROM drive
DVD playback requires DVD drive
Blu-ray™ playback requires Blu-ray drive
For an ATI CrossFireX™ system, a second ATI Radeon™ HD 5850 graphics card, an ATI CrossFireX Ready motherboard and one ATI CrossFireX Bridge Interconnect cable per graphics card (included) are required