PC user thinking of going Mac.. question.

[SixChurchStreet]

I've been using Pro Tools LE since 2001, and always on PCs I've built myself. I've had relatively good stability doing this, but little quirks and an increasingly declining desire to be a computer tech has led me to start looking at the iMacs for my next "upgrade".

My main question is... Anytime you get a computer, you buy it understanding that in X number of years, you will need to either upgrade what you have or buy a new one. If, say, today for example, I bought the 27" i7 Quad iMac with 8GB ram installed, how long (based on what you know about past Pro Tools and Mac software/hardware upgrades) would it be until it's time for a new Mac?

A followup question is... Is the top of the line iMac actually a good choice? The Mac Pros are way out of my price range, this is why I ask.

Thanks!