When you buy a Mac these days, you are paying the premium three things:
1. OSX
2. Cool hardware with cool little extra extra features
3. Excellent customer service/warranty
Other than those three things, the computer itself is virtually identical to a machine you could build yourself. Keep that in mind before you drop the coin.
I run a late '09 13" C2D MBP 2.26, and I haven't felt a need to upgrade yet. However, I don't really use any hog softsynths.
This computer is used for everything except for gaming. (I have an AMD x3 machine for that

)
It really depends what you are going to do. A bit of looping and some light softsynths? Get a refurbed MBP C2D. Heavy DAW work with a lot of hungry software in the studio? Get a Powermac Xeon. Need a metric fuckton of power on the road? Get the MBP i7.
Also, since you're used to Windows, don't rule out upgrading your computer and buying 7. It's MS's best OS since XP (sp3), and actually works pretty damn well.