You know even GTX 460 SLI can run on a 650w psu, you can get this for your build if you want to save a few dollars, XFX and Corsair TX use the same parts. 750 would be better in case you get some more power hungry cards in the future, however. http://www.newegg.com/Product/Product.aspx?Item=N82E168...
also for that budget i would try going for a more powerful gpu, if its just for gaming then an i5 760 should be enough and the money saved by going for a p55 mobo and 4gb of ram and the money saved on the cpu should give you enough cash to go for a gtx 480 or 5970
You 'can' SLI 460s on a 650 watt but it's not recommended. You'll be running close to your PSU's maximum output, so your efficiency will be worse, and you'll be stressing the components by running them full-tilt.
And over time as the PSU's components degrade and provide less power while the GPUs degrade and require more power, it only gets worse.