I really don't use many of the professional programs, except for the occasional TCP, Pixelmator.
My main decision is to buy something that will last a long time or something less powerful that I can upgrade later.
Also, my big fear is the new processors that can be used and since it's already been 9 months since release, then another newer version will come out shortly and have my very expensive computer be an oder version quickly.
There is always a newer version, you can't escape that trap.
If you were considering a 2008 Mac Pro, I would argue its too old. 2009 - 2012 machines are very upgradeable, in many ways.
Even if newer CPUS are more efficient/fast, you can upgrade to a high clock speed used CPU on the older mac pro.
The new Mac Pro is not very upgradeable at the moment, but for an idea of what parts are upgradeable, look at OWC (http://macsales.com
In general, its more difficult to upgrade the new mac pro, and will probably (in my opinion) generally have a shorter lifespan than the cheese grater ones did.
If you do go with a new machine, just buy one. You will need to also buy some sort of thunderbolt storage as well, so factor that into your budget. You will want to buy power you aren't even sure you want, such as going to the six core, and the GPUs and RAM that can't be upgraded. They are amazing machines, just not user changeable.