.... and I am now moving to a full time freelance designer so doing 10hrs/week on an iMac was fine but doing 50+ hours of trade show graphics/web/magazine layout doesn't seem so fun. ....
...Am I nuts for even considering this for price alone or should I continue to drink the Steve-Aid and spend 2x on the MacPro? Decisions Decisions...
Building a computer will take some of your otherwise billable time and turn it into unbillable time, plus you'll eat up much of the savings when you buy windows software to replace what you already have.
And if you're gonna be doing 50+ hours a week of billable time, then you can afford to get the MacPro......but as others have said, maybe consider a used or slightly older MacPro