not too taxing; my friend is playing on a 1.8ghz laptop with an gt*8400 and 3gb of ram and it runs. my system is 2.4ghz) with 4g of ram and a gtx250 and its smooth on the highest settings. like someone else said $500 should cover you just fine.
its an option in the gameplay tab of the menu. it allows you to assign skills from any category to any button; eg. you can assign a "focus" or "technique" monk skill to your rightclick instead of having it stuck on your keyboard. this lets you use multiple skills from the same tree.