It's scalable, so the more power you have, the better settings you can use. It's a case where spending money allows you to buy more frames.
On lower settings it should run on a computer that can run x-plane 9.
The more cores you have, the more it is able to spead flight calculation load and atc stuff without impacting on cpu resources for other parts of the sim, in that with more cores you can turn more features on / up without seeing a frame rate hit.
Remember that while x-plane 10 is being released 7th December 2011 by laminar, I recommend people wait and purchase the Aerosoft European version which should be released in January. This version will be more stable than the 10.0 release and will include a number of European customisations such as autogen, three high quality airports and additional object placement on a number of other big euro airports such as EGLL. That's what I intend to do.
