I have an early 2009 Mac Pro (Nehalem) with 2x2.93GHz processors, 48GB of RAM and an ATI Radeon 5870 video card. I used to have the entry-level GeForce 5200 card but Aperture required a beefier GPU. Even if you don't do 3D, you will still benefit from the GPUs in the Mac Pro as OS X includes OpenCL, which allows offloading heavy computations to the GPUs. Not all image processing programs make use of this yet, but Adobe Photoshop CS6 uses OpenCL for some filters, and Pixelmator does as well.
I will probably upgrade to the new 6-core 3.5GHz Mac Pro (and convert the existing Mac Pro to server duties) once I figure out how to get 2-3TB of SSD storage space for my home directory on the new Mac Pro. Probably using a Promise Pegasus J4 with 4x512GB Samsung 840 Pro SSDs or 4x800GB Intel SSD DC 3500, but I am concerned the J4 requires a custom driver, something which will probably cause compatibility headaches down the road.
The new design is like those computers that get small size by cheating and moving the power supply to a wall-wart. A pro workstation-class machine like the Mac Pro needs expandability, and Thunderbolt options are not quite up to par yet. There are some questionable decisions like having only 4 memory slots, not including 10 gigabit Ethernet, and HDMI 1.4a, while technically giving you 4K capability, only does 4Kp30, not 4Kp60 like HDMI 2.0.
As for the price, pro Macs have a long useful lifetime to amortize on - my previous G5 lasted me 5 years, the Mac Pro will have almost as long, compared to about 1 or 2 years for my HP workstations running UNIX. There are some issues, like one dead USB port, but nothing like the issues that affect my 2010 vintage 27" quad-core i7 iMac at work. The iMac doesn't really have sufficient cooling capacity for the electronics crammed in the rear, and that takes its toll on component lifetime like hard drives.
Another option to consider is a refurb Mac Pro from Apple:
http://store.apple.com/us/browse/home/specialdeals/mac/mac_pro