Recording Amateur Demo, what cheap equipment to use? Options

[frenor]

Hello !

I have searched through this forum and the others but have not yet found the information I´m looking for. Sorry if I´m posting to the wrong forum.

I have a new imac 512MB/60GB and would like to be able to record basic multitrack demos.
I´d like to record vocals, acoustic/electric guitars, piano, and perhaps synthesizer(not recorded from loud-speakers) and drums. It would be nice to be able to record at least two tracks simultanously but it´s not completely necassary.
What equipment will I need to purchase?
I would like the keep the costs down if possible. I´m willing to spend at least 100 USD on a good microphone.
Do I need a soundcard or will a USB converter and a software be enough?

Any piece of advice would be highly appreciated.
Thanks in advance,
Fred