I thinks tom's recently did a comparison that showed in some situations 6 gb was faster than 12gb and that when 12gb was faster it wasn't by much. Also with more ram you can't set the clock speed as high. Really though 4 should be plenty. Though on a 32bit os you will only get 3 gb.
Waste of money if you aren't using what you have now.
More sticks of memory also means more system overhead.
Due to your board design, you might actually see a decrease in performance if:
1. You don't even use the 4 gig you now, and
2. If your board lowers the speed of your memory when you install more than 2 sticks.