Are you talking about crossfiring another of the 7950's you have on that list. If so, the answer would be probably. The tests I've seen set the max draw of a pair of 7950's at 525ish watts. I don't have good numbers for the amp requirements on hand but I doubt that would be a concern with your selected psu. So the answer really comes down to how much power the rest of your system draws. Personally I'd give yourself another 100 or so watt buffer, to account for extra system load, as well as capacitor aging (your psu will deliver less watts the older it gets)
OOOPS. My apologies, the draw numbers I quoted you were for 7970's, not 7950's! Max draw for 7950's is about 360 watts, which is embarrassingly LESS than my single GTX 480 draws! I use a 650 watt corsair with no problems, so your 750 antec should be just fine! You can still up the wattage if you want to futureproof yourself, but as it sits you're be just fine, assuming you don't have a ton of hard drives attached, or are planning on overclocking your system aggressively. Again my apologies for the bad info.
To answer your other question, unless you have significantly reduce the price of your build through amazon (and I mean significantly!), I'd go with newegg exclusively. A lot of times amazon only fulfills your order, with the actual product being shipped and processed from a different company (which may result in a ton of different return policies for different items). Newegg has just about the most bulletproof customer service and return policies out there. I've had nothing but good things to say about the process, and I know friends that have ordered items to have them arrive later that same day!