700w should be fine.
As long as everything fits on the motherboard, the motherboard is fine. A cheap motherboard could have lower RAM voltages which will prevent you from setting the timings correctly, but I seriously doubt that will be an issue. Even if it is, the performance difference is next to nothing.
Once again, though, these are the types of things that should be ironed out already if you're buying a prebuilt machine. The entire point of a prebuilt machine is so that you can have it arrive at your house, plug it in, and have it work with minimal setup. You are ultimately paying extra money so that you don't have to worry about those things.
If you ever have the time, however, it's worth learning to build your own. A few google searches can get you all the information you need to learn how to build a computer, and it'll save you a good bit of money.