The system should use any of the first-generation i3/i5 CPUs (the ones with three-digit model numbers). The second generation (four-digit) i-CPUs won't work, as they use a different socket.

Upgrading the CPU voids the warranty, if that's a consideration.

So any PGA988 would work? but not any i7's? only i3 and i5?

With newer systems that ship with an i3 or i5, an i7 is chancy - many models use a different system board for the i7.  So -- it may work but there's a good chance it will not.

If it is to work, in many cases, you need a higher-wattage power adapter to run an i7 than an i5 or i3.

oh good point, I was just hoping for a bigger cache, all the Mobile PGA988 i3's and i5's have a 3MB Cache. With just 2 cores and 4 threads

The core i7 variants do not have integrated intel graphics - so you'll need an ATI/AMD or NVidia graphics card installed in your laptop to upgrade to i7.

There isn't a big performance difference between the i3 and i5 - they have very similar architecture - the i5 500 series have the best performance as they support turbo boost.

The intel i7 620M or 640M CPU can be used as it has integrated graphics.