memory bandwith is more important to sandybridge than memory timings, within reason.
1866mhz@cas9 and 1600mhz@cas8 will be very close either way(imo bandwith has the bigger advantage). You wont have much opportunity to ramp the ram because of the fairly strict voltages involved with sandybridge. I have the same cooler as yourself and the limiting factor will be what fits under the cooler.
My suggestion would be Mushkin Redline Ridgeback 2x4gb 1600mhz (7-9-8-24), it will fit under the cooler you have, but it is not available at OcUK sadly, and it's going to cost you around £130, if I mispelt area u get the idea
Bear in mind though that 1600@cas9 will be great for your needs anyway.
Note. Just remembered where I saw a nifty table illustrating just what you need to know memory wise, it's
here. About half way down'ish, posted by 1Day. Also check out few previous posts that lead up to it too.
And finally, look at the first page in that thread for the members table. I am ranked at 4th with 1600mhz ram at cas 9 and now take a look at 3rd, Bro306, checking his picture posts in that thread he is running 2133+mhz ram at cas 6. Look how small the difference is. I know Super-Pi is probably not the best way to compare, but if it really was night and day, then i'm sure you would see it in everything.