What ram do i need?

DDR3 does not need to come in any sets (2 or 3). The Core i7 920 supports tripple channel, and therefore, you want sets of Threes to get the most performance out of your system. If you had a Phenom 2, or Core i5, you would still want/need DDR3, but in sets of twos.

As for speed, get whatever is most affordable. 1333 or 1600 are common and both are fast enough. You can overclock the memory to get better speed out of it if necessary.

This article will give you a good idea of any differences: http://www.tomshardware.com/review [...] ,2372.html

EDIT: Your twos versus threes comment was about the number of sticks in a set right? I suppose it could be about the speeds too, but all DDR speeds have the same base, be they DDR, DDR2, or DDR3.