Under preferences, there is one called energy saver, you can set the monitor to sleep independantly of when you want the hard drive to sleep. So if you want to keep things running at night but dont want to burn out your monitor's lifespan too quickly, it might be a good idea. It should wake from sleep in a 2-3 sec of pressing a button on the keyboard. One important thing about leaving you computer on 24/7 is that osx will run some regular maintence apps in the background at like 3 or 4 am every x days or weeks (I can't remember the specifics). So it might be good idea to keep it on overnight every now and again (1per month or something).
Another thing is if you ever start experiencing random quirky behavior, you should go into your apps folder, and then into your utilities folder, and open disk utility. This app allows you to "repair permissions" and you can think of it as defragging if you like (even though that's not what it's doing, mac's don't need defragging). That's always a great first stop for trouble shooting.
You might want to avoid having too many 3rd party apps/betas, at least at first until you get comfortable w/ how things run, b/c somtimes they can cause weird problems when Apple releases a security/regular update, so you might want to hold off on those for a month or two.
When the scroll ball on your mighty mouse gets gummed up (and it will), flip your mighty mouse over, get that soft microfiber screen cleaning cloth and rub it vigoriously back and forth over the mouse ball (it works like a charm for me).
Get a book like "the missing secrets of tiger" there are lots of em like that, and read up on all the cool little things that apple's worked into the os that most people never noticed. I've been a mac user for as long as I can remember, but I bought one for 10.2 and was astonished about how many things were in osx that I never knew were there (I'll help you pass the time while you're waiting for your order to arrive).
Feel free to ask us here at MacRumors anything (try doing a search first though cause sometimes people get pissy w/ repetitive threads), It's a great community, full of smart/friendly/helpful people. Oh and don't let all of my suggestions scare you, you can probably ignore all of them and never have a problem (osx is rock-solid), but it's always good to play things safe at first (just my opinion).
Oh, and you should be sure to let us know what you think about the new iMac! Many of us were drooling over it and are very jealous, so we like to enjoy new macs vicariously
. We also love switch stories, so feel free to share yours.