The voltage switch is just a cheap way for PSU makers to use 110 and 220 volt outlets. Better quality PSU have more expensive parts in them that work for both (little circuit board). What is your second GPU that you got? I would recommend getting one of these that have 4 PCI-E connectors and you don't have to use adapters. When you buy a PSU there are only certain brands I even consider, a few are OCZ, Antec, Corsair, SeaSonic, PC Power & Cooling, XFX and some Enermax. For your price range the cheapest are these http://www.newegg.com/Product/Product.aspx?Item=N82E16817207014http://www.newegg.com/Product/Product.aspx?Item=N82E16817371051http://www.newegg.com/Product/Product.aspx?Item=N82E16817341041http://www.newegg.com/Product/Product.aspx?Item=N82E16817371049
You could get the 650w depending on what cards you have. If you get a 750w consider one that is modular for nicer installation. You NEVER want to cheap out when you get a PSU because if it goes bad it can fry other computer parts, potentially making you lose hundreds of dollars instead of paying a few more for a good model up front.