I'm not exactly sure, Scott.
You can set the iPod to to download new sermons automatically, and you can also set your iPod to delete the ones you've already listened to (but it will still be kept on your computer's hard drive on iTunes-- so you will have a permanent, ongoing archive, and you can drag 'n drop other sermons back onto it, if needed).
Because of that functionality, I can't imagine you would need more than a 2 gig, which can be had for very little $ now, but they have a 4 and 8 gig also available.
I have the 30 gig video iPod, and I have 3 two-hour movies on it, virtually all the music I've ever loved, pictures of my family, all the email addresses I have in my Outlook express folder, and the following podcasts (three months of each, with sermons broken down by the half hour for most):
1)
Renewing Your Mind, with RC Sproul
2)
Grace To You, John MacArthur
3)
Desiring God Radio, with John Piper
4)
Truth For Life, with Alaister Begg
5)
Christ Covenant Reformed Presbyterian Church Sermons, C. Matthew McMahon
6)
The Wild Boar Newscast, C. Matthew McMahon
7)
FBC Boynton Podcast, (my church's sermons)
Nearly all of them upload daily, so I never have a shortage of good stuff to listen to in the car, on the way to and from work. I also subscribe to some goofy video podcasts, and I have it set to KEEP THEM ALL in my iPod.
The crazy thing is-- I haven't even used HALF of the harddrive in my iPod-- I could DOUBLE what I already have, and still have room!!!!
Kinda hard to lose-- at least, that's how I feel about it.
Get ready, though-- when you hear about 2 to 4 sermons a day, like I do, you will REALLY be alien to this world!!!
Alex