You don't need that spec of memory if you're looking for the type of overclock you mentioned in your motherboard post. Total overkill, plus you'll typically get tighter timings on slightly lower speed DDR3 as they tend to be a bit more generous at the speeds you're looking at.
4.2Ghz is a breeze with DDR3 1866Mhz, and you probably don't even need that.
These are 100 quid and what I use :-
http://www.overclockers.co.uk/showproduct.php?prodid=MY-066-GS&groupid=701&catid=8&subcat=817
Incidentally, I don't want to harp on about it but if things like memory timings and tuning are that important to you then the formula would have been the better choice, especially if you're hell bent on getting the fastest memory you can lay your hands on.