Right on the product page it shows the following:
"4×240pin DDR3 2450(OC)/2100(OC)/1600/1333/1066 "
So, any ram that fits those speeds will do, specifically 1600, 1333, and 1066. If you pick 2450 or 2100 (or otherwise) you will have to overclock the RAM to run at those speeds.
Generally, the higher the speed of the RAM, the more expensive it is. I say go for at least 1600, that stuff is cheap, and should be guaranteed to work.