Is the intel i3-2120 much better than the intel g620 processor

Yes and no. The answer to this question has a lot to do with what you are doing with your system. With the Intel® Core™ i3-2120 you have a processor that is 700MHz faster and has hyper-threading. So if you are doing something that takes is processor intensive you will see a difference between them. However if you are just doing internet and word processing you most likely won't see much of a difference. Also if you are using the IGP (Integrated Graphics on Processor) you can may see a performance difference there as well.