It depends on what you want to do. In general (for the desktop), i3 is a dual-core hyperthreaded chip (2 physical cores each capable of 2 threads simultaneously). The i5 is a quad-core non-hyperthreaded chip (4 physical cores each capable of 1 thread). The i7 is a quad-core hyperthreaded chip (4 physical cores each capable of 2 threads simultaneously). You can sort of think of hyperthreading as adding an extra 25% more performance per physical core. In terms of performance: i3 < i5 < i7. For most uses, an i5 is plenty.