I'm sure you don't want to hear this, but I think you're better off with an iMac or a mac mini with a nice IPS monitor. I understand that the mac pro is easier to expand, but if you plan on keeping it for 4 years you could buy a new mac mini every year and still come out the same.
I just saw this, and I have one point of contention- a Mac Pro can have multiple quad core processors- Minis have a single dual core processor. Depending on the type of photography you're doing, that can make a serious difference- for example, focus stacking, HDR and panoramas will all benefit from more cores. I mentally kick myself every single time I go to stitch a 20+ shot panorama because I "cheaped out" and only got a single QC processor in my Mac Pro.
Paul