Hello.

I am interested in how much does caching really enhances the site performance. Can it really a big difference? Is it important?

You can estimate the percentage of around how much does the performance usually increases, but as far as i know i don't see any notable increase, but i'm on modem and i am i so somebody might actually notice some increases..

The reason why i am asking this is that caching takes up alot of space (if you turn caching on to all modules it actually copies the site so it always takes twice as much space as one whole site setup).

That's why i like to keep most of caching turned off, at least until i get to buy a gig of hosting

Thanks
Daniel