Actually I have no preference. I've used both Nvidia and ATI, I like both companies and think the only reason they're not teaming up is because that would create a serious monopoly and they'd get too rich.
You should definitely go with the X58 then. Sabretooth seems like the best bang for your buck
Sounds about right. Just remember the Gulftown series of processors are known for drawing immense amount of power, as is the 580. You should get at least a 750W PSU.