the i7 is a quad core with hyperthreading. either s1366 which uses an extra chip (a stripped down northbridge) to get 2 slots of 16x pciev2 for video cards. or the s1156 that will have one 16x pciev2 video or two 8x slots. by fully integrating northbridge into CPU. (aka, s1366 gets better two slot video card performance, s1156 gets BETTER single slot video card performance. due to lower latency of having the video card connection come out of the cpu)
the i5 is either a quad core without HT, or a dualcore with HT.
the i3 is like the i5, only without turbo mode.
so... what programs do you run that make use of 8 threads /cores AND benefit from HT? (some lose performance on HT)