Considering that the Core i3 is four-threaded and only lacks the Turbo Boost feature compared to the Core i5 and i7, the i3 is still faster than the vast majority of Core 2 Duo’s. But if you want the best you want the best.
Tom’s Hardware ran an interesting comparison the other day that measured the actual difference in gaming between the desktop version of the Core i3 and a hanful of other CPU’s – all with the same ATI Radeon 5850 GPU – and reached some interesting conclusions. Of course, desktop and laptop CPUs are different breeds (not to mention GPUs, which are downclocked in their mobile versions), but on the whole the results should be relevant.
They tested Call of Duty: Modern Warfare 2, Crysis, DiRT 2, Far Cry 2, HAWX and Stalker: Call of Pripyat, and the i3 dosn’t cripple gaming performance by any significant amount, even with a new and powerful GPU like the Radeon 5850. In none of the games was the i3 outperformed by more than a handful of frames per second.
In other words, if playing games is all you do on your laptop, upgrading to the most expensive Intel CPU is unlikely to help your games run noticeably faster.