what is the difference in the performance of i3 and i5 and i7. is this difference is very big or not? for multitasking,playing 1080 movies,surfing internet with 50-60 tab and small video editing software - which processor should I buy?
i5 and decent GPU I would think, depends, if you are happy to scan around places like eBay and pick up a used i7 and a used high end GPU then you can achieve a higher spec system for less money, as long as you know what you are buying.
The difference between the i3 and the i5 is huge. You'll notice it all the way around. However, the difference between the i5 and the i7 is much smaller. You'll only see a difference if you do very processor intensive things such as video editing or workstation programs (IE Inventor/CAD stuff). For gaming and general use, the i5 is the one you want.