Check out my build guide in my siggy for a bit more suggestions.
You should look at an 8gb kit. You don't need CL7, CL9 is good enough and the speed differences are hardly noticable.
That site doesn't have the Extreme4 but this would be a better overclocker than the board you chose. A little more but hey, better OCs.http://www.overclockers.co.uk/showproduct.php?prodid=MB-044-AK