go 750w and give yourself some headroom although a solid branded 650w would suffice, i run my sig rig on a coolermaster 520w with no probs as quality is better than quantity but i still plan on upgrading to give myself some headroom.
A loaded rig with 2x GTS requries 500W PSU with 30A on 12v rails. With that says, A good 550W PSU should be fine. So, depending on your budget and future upgrade, $70 Corsair 550VX, $100 Corsair 650 or $150 Silverstone DA650.
Yea the Corsair 550VX or 650TX are good options, but there's also a good deal on the OCZ GameXstream 700W selling for $95 after MIR.
All three would work just fine. The Corsairs come with a 5 year warranty and a strong single rail (I prefer single rail). The OCZ comes with a 3 year warranty and 4 rails. The Corsair 550 only has 1 PCIe 8-pin connector and 1 PCIe 6 pin connector, which is fine for your current setup, but if you ever upgraded to SLI a pair of video cards that each required an 8-pin connector you wouldn't be able to do so (and it would also be pushing the 550W capacity). So the 650TX and OCZ 700W would future proof you a bit more, but the 550VX is plenty for your current setup.
Also, make sure you get your Corsairs off buy.com if you choose Corsair; they're much cheaper there than newegg.