Intel Corei5 or i7 turbo boost?

Turbo boost is minor automatic overclocking on one of the cores when CPU-demanding tasks are encountered. If several cores are used, turbo boost won't be as effective. Laptops are far from my realm of expertise (I have but 20 total hours of use versus tens of thousands on desktops) so I can't answer those questions.

but it has 4 cores!!!

This topic has been moved from the section CPU & Components to section Laptops & Notebooks by Mousemonkey

The main advantage of some i7 over i5 is it have 4 physical core and hyperthread where as i5 and the "lower powered" i7 only has 2 core and hyperthread.
If you use a lot of program that are well threaded and can take advantage of more core then the i7 will be faster.

It depends on whats your intended use? what kind of program you want to run?

If you run multi threaded apps, then 4 cores is an advantage.

Few games use more than two or three cores.

Normal desktop usage is perfectly happy with two cores. Actually, the i5-2430M also has hyperthreading which gives you two extra tasks that make use of residual cycles in the two cores. Think of them as the addition of 1/4 of a core each.

Also, the more cores you have, the less powerful each core can be and still be within thermal and power limits. It is a trade off. With turbo boost, if you are running a single cpu intensive task, the cpu can go faster(turboboost) since the power for the other core is not needed. It is a good thing.

For the most part, ignore l1/l2/l3 cache as a selection criteria. It is an appropriate amount for the cpu, and is already factored into the capability ranking of the cpu.