First off, Welcome to the 'nova. Second - as far as first posts go, this one is top notch - so well done there!
Third, I would recommend the 24" iMac over the 20", assuming you can, and are willing to afford it. I'd stick with the 2.4 Ghz model and upgrade to 2GB of RAM (that's a $150 upgrade from Apple, or you could possibly do it for less yourself). Jumping from 2.4 to 2.8 Ghz will not be terribly noticeable, and certainly not $250 worth of noticeableness (please forgive generous abuse of the language).
Which version of MS Office/word do you have? As for the other software, I don't know right off whether it would work or not (with the exception of iTunes, which will already be installed - you'll just need to transport your library over and that should be simple with the built in migration tool). If Chucker happens to wander into this thread, I'm sure he'll know about the rest, as might others.