Yes, Applecare is worth it, because macs do break from time to time and then it gets really expensive. My first iMac had problems after 3 weeks.
Though I would wait to get extended warranty until the initial warranty is almost over (because maybe you decide to sell it or whatsoever). It doesnt hurt you in any way. Initial warranty should cover 1 year, I think.
Buy Applecare on EBAY (!) not through apple. That saves you about 300$ or more.
The current mighty mouse is crap, if you ask me. Its a real good mouse, but the trackball attracts dirt like a dirtmangnet. After a while it gets stuck and by that time you got so used to it that it bugs the hell out of you.
Software: Transmission, Final Cut, Adium, Office for Mac 2008, Skype, iLife, Gragebuy or iSale.
DO NOT buy RAM extensions from apple. I reapeat, DO NOT. I remember someone here spending a fortune on it. You can get 1GB of RAM for about 10Euros. Apple will charge you 100 or more!
iMac is great, but be sure to buy the big one, the smaller one has an inferior LCD screen which is really ugly IMO