Lol, no problem.
i3= low power,somewhat fast, everyday use
i5= high power, everyday use, multitasking, fast
i7= extreme power, heavy multitasking, everyday use, huge workloads, future proof
It pretty much goes like that. I would reccomend the i5 for you because seeing as your a student, you probably want something with power and to be a somewhat future proof. it also comes with a 27" 2560x1440 resolution screen, which is an insane resolution. the screen on the 27" imacs cost about $1000 alone
As for graphics, it goes like this: ATI Radeon HD 5750 with 1GB, with 512MB ATI Radeon HD 5670 with 512MB, and ATI Radeon HD 4670 with 256MB. Those are offered on the current imacs, with the ATI Radeon HD 5750 being the most powerful graphics card offered with the iMac.
I would really recommend looking at the refurbished store. Now you hear the words "refurbished" and you think used. Not true at all. Most of the refurbished section is just things returned within 14 days and they go through rigorous testing processes at Apple, with the keyboard being replaced, the mouse being replaced, and the whole aluminum outer shell of the iMac replaced.
If you put a refurbished iMac next to a new one, you couldn't tell the difference. I have a refurbished iMac and I would only buy refurbished macs now because they are just like new.
If I were you though, I would wait. The iMacs are at the end of their update cycle, meaning that new models are coming out very soon and they will probably have a better graphics card.