You primarily need to keep track of the sockets, there is the LGA 1366, which takes the i7 920, 965, and 975. There is also the LGA 1156 which takes the i7 860/870, the i5's and the i3's.
The i7's are the best as they have 4 cores, with hyper threading which gives them 8 logical cores making them faster in multi threaded tasks but they are also more expensive.
The LGA 1366 socket only has 1 chipset for you to pick from, the X58, the LGA 1156 gets a bit more confusing with the P55, and the new Hxx chipsets, the Hxx's offer the ability to use the IGP on the new i5's and i3s but the p55 gives you the option of crossfire/sli. Really it depends on what you want to use your system for, if you are trying to find out what one is best for you, post your budget and your intended uses and we can help you figure it out.