The ModXstream has insufficient power connectors for your planned SLI upgrade. Both the XFX and the PC P&C have are electrically sufficient with 53A on the 12V rails and sufficient power connectors. Both are good quality and I recommend either one for your use assuming you are definitely going to SLI. If you're not going to SLI, then I would recommend the Seasonic M12II-520: http://www.newegg.com/Product/Product.aspx?Item=N82E168... It's more than sufficient and is a great PSU; I'm using one for my more power-hungry HD 6950.
Am I being paranoid? If I decided to get an SSD later, would the 520 still be able to handle it? And can I get some good overclock on it? Sorry, I just want to make sure I don't screw up something.
A good kind of paranoid, but yes you're being paranoid. An SSD has a minimal power draw; less than 10W at max load. For a system with a single HD7950, the M12II-520 will have sufficient headroom for significant OC of the CPU/GPU.
The M12II-520 is around $3 more. Same warranty but the TP-550 gets you 5A more while the M12II-520 is modular....you don't really need the extra amperage or the modular cables so choose based on which mfr you like better and whether or not you want the modular cables or the extra amps.