Recording a guitar on iMac 2012 model.

I have an old Macbook (which still works surprisingly well!) from 2007 which i have been using to record various guitar/bass parts. Having purchased a new iMac (late 2012 model) i notice it has no mic input (which is what i have on my Macbook) which is how i have been recording up until now.

After trawling various message boards and forums it seems it is either a cable like this: http://www.amazon.co.uk/UBI-Soft-Rocksmith-Real-Cable/dp/B007KGISK6/ref=sr_1_1?i e=UTF8&qid=1363271329&sr=8-1

Or an interface (d?) one like this: http://www.amazon.co.uk/Griffin-GC16031-Quality-Recording-Playback/dp/B000BVV2IC /ref=sr_1_1?ie=UTF8&qid=1363270732&sr=8-1

I have to say, i'm no professional music dude, i just do it for fun. I also ideally would like to just plug in the guitar to the computer and go.

However, because i record rather amateur bits and pieces i don't want some expensive rig that's £250+. Preferably something for the prices above that allows a decent record quality that will sound good when loud.

As i cannot seem to find one unifying answer, i'd really appreciate some tips from you guys.

Thanks.

Hen.