How much difference would be between and i3 and i5 processors.
Essentially the I3 I5 I7 processors have caused a lot of confusion and there are a tremendous number of factors involved in answering your question.
Is an i3 like the old Celeron's?
No, it is nothing like a Celeron. The I3 is a dual core processor. But there are 2 different I5 processors as well, and one of them is dual core and the other is quad core.
Computer would be mostly used for email and web surfing.
Depending on the price break in your market, the I3 would be fine for this type of usage. If you can get an I5 for $20 more, by all means grab it...
If you would like a better explanation and help in making your decision, check out this very good article that explains it in more detail.