Simple Audio setup for new iMac

I recently purchased a late 2009 i7 iMac and would like to use it for (among other things) some light audio recording (recording saxophone). I would like some suggestions on a relatively inexpensive audio interface (I probably don't need more than 1 XLR input) and advice on how to setup my computer, if anything special is needed.

What I plan to be doing is either creating or (more likely to start) using pre-made background tracks (by importing mp3s or other audio tracks or perhaps midi files) and recording on top of that (one or more tracks), then adding some light effects and mixing. I will likely use Garageband to start and see if it is sufficient for my needs and upgrade to something else later IF I find it necessary.

As far as set up, I was wondering if it is necessary to partition my hard drive (I have a 2 TB drive) so that the data files are saved on a different partition from where the program resides, or if I should otherwise use an external hard drive.

Any other suggestions or advice is appreciated.

Thanks for the response. I was looking at the Apogee products and they seem like pretty good options. Do you have any idea how they would compare to the Presonus Firestudio (mobile) or one of the M-Audio interfaces? The only thing that concerns me a little about the Apogee units is the fact that they are useable only with a Mac. Now that may turn out to be a good thing because of the integration, but it could potentially be limiting down the road too. Perhaps I shouldn't worry about that...