Here's what I use: http://www.fatwallet.com/Newegg-coupons/g-skill-ripjaws-x-series-8gb-2-x-4gb-240-pin-ddr3-sdram-ddr3-1333-pc3-10666-desktop-memory/.
The 1866 is faster ram, but only works at the higher speed when overclocking the cpu; don't know how stable it will be when overclocked. You have to experiment with the voltage and ram settings when overclocking. Sometimes you get a complete failure or at the very least, have to reset the bios and start over. If you don't have the patience for it, save some $$ and get the 1333 gskill.