Given the same clock speed, Ivy Bridge is faster by 5% on average so an Ivy Bridge i7 running at 4.0GHz is basically equal to a Sandy Bridge running at 4.2GHz.
Ivy Bridge uses less power. On a full load, Ivy Bridge uses around 15w - 20w less power.
PIC-e 3.0 is supported by Ivy Bridge. It offers more bandwidth for graphic cards, but no current graphic card is limited by PCI-e 2.0's bandwidth yet. Maybe two graphic card generations from now a $400+ "Radeon HD 9xxx" or "GTX 8xx" graphic card will begin to be limited by PCI-e 2.0.
Ivy Bridge CPUs does not dissipate heat as well as Sandy Bridge CPUs. When not overclocking, it'll be fine. The heat prevents it from being overclocked as high as Sandy Bridge. But this is offset by the average 5% increase in performance.