For sli gtx 560, minimum would definitely be 650W.
For sli gtx 550Ti, you could go lower than 650W.
Palit GeForce GTX 550 Ti 1GB Sonic Video Card in SLI Review - Power Consumption Tests :: TweakTown USA Edition
For sli gtx 550Ti, you could go lower than 650W.
Palit GeForce GTX 550 Ti 1GB Sonic Video Card in SLI Review - Power Consumption Tests :: TweakTown USA Edition