Composing on the Mac is wonderful. I've been doing it for a long time. I've done it on the Windows side as well, so I can certainly appreciate the differences.
One thing you need to be aware of is that the Universal Binary version of Kontakt has not been released yet. What this means is that if you purchase an Intel Mac today and install Garritan Personal Orchestra (which uses Kontakt Player), performance will be pretty lousy. While Apple's Rosetta emulator works surprisingly well, audio applications are one area where it starts to falter. Native Instruments says that Kontakt Universal might be out as soon as October, so this might be a non-issue pretty soon.
A side note: I don't know if you are interested in using Logic, but it's incredibly fast on the new Intel Macs. It was never slow on the PowerPC, but on a Mac Pro, Logic is really something else.
Sibelius 4 is not a Universal Binary either, but runs extremely nicely on Intel Macs, so it shouldn't really be an issue.
Since you already have a system running OS X, you probably already know how much this wonderful OS likes RAM. For what you're doing, I would not go below 2 gigabytes of RAM. If you're planning on upgrading to Garritan Advanced when it is released, even 2 gigabytes of RAM might not leave much breathing room. As it is, I find 1 gigabyte of RAM to be the minimum for basic "messing around" in OS X (Web browsing, email, office applications, etc.), so for audio, make sure you budget for extra RAM.
Without knowing how serious you are, I can't really tell you which Mac to buy. But I know plenty of talented, productive musicians who are turning out great stuff on their iMacs. You don't have to get a Mac Pro, and unless composition is your livelihood, I would recommend against it.
My 2 cents.