what is the difference between i5 and i7?
The i5 series CPUs all fit onto P55 motherboards (or the lesser H55 boards). Most are Dual-Core, however the i5-750 is a fantastic Quad Core CPU that OC's very well for gaming purposes. The i5 series are typically less expensive (as are their motherboards). They are designed for Dual-Channel Memmory (DDR3 type).
The i7 series can be tricky. There are a couple models (i7-860 for instance) that work on P55 motherboards. Most i7's work only on the X58 chipset, and use Tri-Channel Memmory (DDR3 type).
The big difference for the most part (at least with the quad cores) is that the i7 chips have Hyperthreading ability. The i5-750 does NOT have hyperthreading. Hyperthreading allows each CPU "Core" to act like two processing units IF the program being run is compatible with Hyperthreading (almost no games support this).
Other than hyperthreading, they operate with the same basic architecture. Hyperthreading and dual vs tri-channel memory are really the only 'real world' differences.