Once you know the video card you can determine the size of power supply you need. I suspect it will be lower than 750W.
I can neither confirm nor deny the compatibility of the i7 870 with that motherboard, but I don't see any reason why it would be a problem. You may well want to use a non-standard CPU cooler, but you will have to assess which coolers will fit into the machine.
If you have room, I would suggest considering a Noctua cooler - they make excellent coolers that run fairly quietly. One site that has some excellent reviews of CPU coolers is http://www.silentpcreview.com - they have reviewed most of the Noctua coolers (and a great many others as well.