Would 550W be bare minimum? Could 550W handle minor upgrades in the future?
Yes, it could handle more hdds or ssd, or even a newer card, you can expect the future card released with even lower power consumption, so it won't be a problem.
Those xfx are quality psus.
But you can grab the xfx 650w it's at a good price, if you want to.
JohnnyGuru rated the OCZ at an 8 which is not that great.
HardOCP gave it a pass, which is not great.
The Seasonic G i linked was rated 9.6 at JohnnyGuru. The Seasonic X series have gotten must buy and gold ratings - they are excellent. I think the only one I listed that the OCZ might best is the Seasonic M12II.
Nobody has mentioned this. But if you plan to SLI, then you want a PSU with 4 PCI-E connectors. Most 50xx W supplies only have 2, including the two I linked above. You would need to use a Molex to PCI-E adapter for the other two - the graphics card will come with these.
It isn't until you get into the 650W supplies befre you get 4 PCI-E adapters. The two I linked in above have 4.