I was in a similar situation as you awhile back. I was always a windows person, My main computer was running Windows Vista. My dad has an Imac running Leopard so every time that I was over his house I got to use it.
Lets just say that I loved coming over his house! There was something about using a mac that was much more enjoyable. So I decided to get a mac, but which one?
A Mac Pro or an Imac? I did some research and was told that for what I'll use my computer for then an Imac is all that I need. So I decided on an Imac.
I used it for 1 week, after that I returned it and purchased a 2.66 ghz Quad Core Mac Pro which I've been very happy with every since. Why did I return it?
Well the first reason is because I'm a person who likes a speedy computer, even if I'll never use it to it's full potential I like it powerful.
The Imac was a bit too slow for my taste. It became sluggish at times. The second reason is because it's not upgradeable like a Mac Pro, I like having that ability.
The Third reason is just a personal taste but it was important to me, I like having a tower attached to a monitor as apposed to a computer built into the monitor.
I've got a stunning 24" Apple Cinema Display to go along with it which I love. To be honest I had always had my heart set on a Mac Pro.
I would go with what you would be most happy with. Don't let other people persuade you from getting a Mac Pro if that's what you really want.
Don't let people tell you that if you'll never use it to it's full potential then you don't need it (like in my case).
Hope that helps!