I am currently running Apple Garage Band on a Mac Book Pro that has a 2.2 Ghz Intel Core 2 Duo processor and 4 GB of RAM. I have never had a problem running as many audio or MIDI tracks as I'd like in Garage Band with this set up. I have run up to about twenty tracks of audio or mixed audio and MIDI at once without issues. However, how many tracks and effects you can run at once really depends on your machine more than the software itself.
I've been using Apple Garage Band for around year since I got my Apple computer that it came installed on. While I have Apple Logic and Pro Tools that I use for most of my projects, I use Garage Band for demos and ideas when I just want to record straight from my computer microphone. The price of Garage Band is free with an Apple computer, but is cheap within the iLife bundle if you need to buy it. Garage Band is powerful software even though it comes with all Mac computers. It is great for beginners looking to try out this type of software.