Too early to tell how much of an improvement Ivy Bridge will be over Sandy Bridge. At least on the CPU core side. I'll give a conservative estimate of 6% +/- 1%. An article from Jan/Feb 2011 estimates up to 20%. An article also from the same time period stated that Bulldozer would be up to 50% faster than Intel's Sandy Bridge CPUs...
However, Intel has emphasized that the graphics core will be much improved. Anandtech believes the Intel HD 4000 will be up to 60% faster than the Intel HD 3000. If true, that's bascially going from the Radeon HD 5450 to the Radeon HD 5550.