Hmmm...
1 - Get a 2-button mouse, run, don't walk.
2 - Calibrate your screen in system preferences. The iMac has some trouble with antialiased fonts being fuzzy, this is made worse because the iMac ships at a default gamma of 1.8 which I think is ridiculous for a general purpose PC even on the "platform of graphic designers." Pieces of aliased fonts and a lot of color in OS X is washed out with a gamma this low.I recommend the "PC gamma" of 2.2 or {higher}. Your eyes will thank you and OS X will look even better.
3 - Learn to use the Dock. It can be as functional as the Start Menu and much more fun. I would recommend putting your Application, Documents, and Music folder there.
4 - Remember when you close the last document, the application is not closed. You can Apple-tab to browse through open apps, and switch to Apple-Q without releasing the Apple button to close from the Dock. I recommend auto-hiding the dock at the bottom and Apple-Option-D to turn hiding on and off. It's cool to have it on when playing, but when working you'll want to hide it. All apps share the Finder menu as their main menu, and with the Dock then being at the bottom of the screen, it takes up too much screen real estate.
5 - The biggest thing you'll miss is all the time-saving shortcuts of the pervasive context-sensitive menus in Windows. Like me you'll just have to deal with this one, it's a big one, and wait for Apple and Mac ISVs to catch up.
6 - Don't be afraid to complain about the good things in Windows missing from OS X. There's a better chance Apple may add them to OS X than there is of Windows ever being this cool.
7 - Have fun, it's the biggest advantage OS X has on Windows XP.
8 - Almost forgot, if you need a small pair of desktop speakers, the Apple pro speakers are amazing for their size.