Recommended Setup

Before you settle on Logic, I'd suggest you investigate some alternatives.

I'd suggest you look into Cubase. If you plan to do much in the way of editing (after tracking), Cubase offers editing abilities that are far ahead of Logic. After trying out several different apps (including Reaper, Traktion, and M-Audio's new app), NOTHING I've seen comes anywhere near the ease-of-use of Cubase, insofar as copying, pasting, modifying, and moving audio clips is concerned.

While you're playing back your track, have a look at the activity monitor. Especially the CPU and System Memory tabs. Look to see how much %CPU GarageBand is using and then check how much memory it is using. Also check how much Page Outs there are.

You computer needs depend on what you're trying to accomplish. If you're recording multiple tracks at once, then your harddrive will probably be the problem. If you're using a lot of synths and effects than the CPU will probably be the problem. If you're mixing hundreds of audio track than it might be ram.

I was using Logic on my '08 macbook with a 2.0GHz core2duo and 2GB ram and I wasn't having issues, though my projects were much smaller back then. The important thing is that Logic doesn't really need more resources than GarageBand to do the same job. You just start having more complex mixes in Logic which changes things but that's a whole other story.

I think your computer will be plenty fast to do basic mixing. I would look into getting 8GB ram mainly because it's so cheap (you can probably get it for 40$) and could be helpful if you're leaving other programs open with Logic. You might want to have a at your hard drive if you're recording several tracks at the same time.

As for other DAWs, I think Logic is the best deal to be had right now, especially if you're graduating from GarageBand, you should feel right at home. The included effects and instruments are very very good considering the low price. Nothing wrong with checking out some of the other DAWs available though.