Being a former Windows guy myself, let me say welcome to the Mac clan. It took me about a week of completely immersing myself into the OS before I could rate myself a power user. Everything had a fast ramp up and I was completely void of any desire to go back into the Windows environment within a month.
Just to let you know that I’m running on the same iMac hardware that I purchased FCS1/Tiger on. Today I’m fully loaded with FCS3 (FCP 7.0.1)/Snow Leopard (10.6.1) and editing everything from DVCPro50 to all the flavors of ProRes, AVC-Intra, and dealing with a P2 workflow seamlessly.
Today’s Mac Pro will get you speed, but not more utility. I’m just now starting to feel the pinch of speed restraints, but still I’m quite productive even while running other things in the background like Photoshop CS3.
Whatever you do, make sure that you get yourself a minimum of 4G of RAM. This is where I’m most limited since my iMac can only hold a max of 3G.
All your hardware answers are dependent on how much coin you can toss into the venture. If you’re looking for an entry machine, I suggest going the iMac route which has the monitor built in. If you have the cash and you know you’re in for the long haul, any of the Mac Pros will work.