Once I hdinstall my PC, all I need do is run apt-get update every week or two and I never need to download another ISO to stay up to date, right?
Hi,
If you don't care about updating your kernel, apt-get is all you need.
Someone please prove me wrong, 'cause that's what I believe

BTW, what's the difference between update and upgrade?
update updates your packagename and -version database to know what upgrade will need to upgrade. Hmm, not very clearly put, better have a look at $ man apt-get

how do I know what's available out there to install
You mean all the debian packages, or new versions of the ones you have?
I'd normally browse at http://packages.debian.org/testing/
but it's down for the moment
Try $ apt-cache search packagename/part_of_a_packagename

You can also do $ cat var/lib/dpkg/available

Man pages tell you a lot