Congrats on the purchase, I doubt you'll be disappointed. I can only speak of my experience. I prepared myself for the big shift and found it to be astoundingly easy. I prepared to load both windows and microsoft office on my mac and as of yet haven't found it necessary. iwork is a must in my opinion. It's $70 should you choose to buy it and I prefer it greatly to office. You can save and open any file from office, so compatibility issues aren't really a problem at all. I certainly wouldn't get them both.
ilife is free with the mac, so don't worry about buying that. A ton of functionality there.
As for flac, i can't help you there at all but it looks like cliche has you covered there.
I made a "getting a mac, what do I need to know" thread a while back and from it came the advice to download the VLC player. Seems to play all sorts of media and saved me a lot of headaches.
If you're curious this was my thread:
http://forum.calgarypuck.com/showthread.php?t=60229