The big thing with podcasting is ambient noise, i.e. the sounds all you that you never hear unless you listen to them recorded on your computer. There's a static-ey feel to the podcast that really puts people off.

I bought a high-end soundcard to do podcasting, called an Aardvark (Guitar Center has them) along with a high-end Shure microphone. The sound quality was no better than the $10 microphone I bought at Circuit City.

Now, I was just using the sound recorder that ships with Windows to do my podcasting, and putting the cast through a noise gate was what got rid of the static. Downloaded Audacity off and have found no reason to move off that.

I guess much depends on the cpu you are going to work on. Audacity is more versatile, and thus works well with Windows/Linux, GarageBand is an Apple product and designed to facilitate files on Macs. I use Macs, thus i like GarageBand; it came with my system.

As Mike points out, you can spend a hundreds of dollars for high end mics and get nothing better than you get with a $10 radio-shak model. More important is the way in which you control the sound on your computer, with filters and higher quality sound editing products. Herein lies the response to your third question. To make a five minute piece that you are really happy with and that you have run through enough times to insure its quality and static cleansed nature, it will take you much longer than you think.

The podcast audience is essentially split between those who like to listen to audio books and those that will tune in for the tidbits between the music. Who is your intended audience and to whom would you most like to receive feedback and response from. If you work through a dozen or so five minute pieces, you will realize just what you are best at delivering and editing. You many find you can deliver a fantastic fifteen minute pop, but you might also find a three minute one is totally sufficient and greatly appreciated.

My experience is in packaging soundscapes for various conferences and workshop sessions. I work from analog inputs into a digital format that i can edit on my Mac. The expense of A:D interface is dramatic and for the most part an indulgence. I don't recommend it unless you also have the need. I would think you could find a great DAT deck relatively cheap(very relative here) if you want to create away from your desktop/ laptop and then download onto your hardrive to work on later. A good DAT deck or harddisc recorder and a pretty good mic lays down some surprisingly good sound without much static buzz and noise.