I'm running the early 2007 MacPro 8-core with 32GB RAM, OWC SSD boot drive, 2x1.5TB Raid0 and 2x2TB Raid0. The OS has been upgraded to every new release that has come along since I bought it and is currently 10.7.4. It's my main Lightroom/Photoshop workstation. I have 2 external eSata 5 bay SansDigital Raid towers connected via a Highpoint eSata card.
I've upgraded the graphics card to the ATI Radeon HD 5770. It very fast and, more importantly, it's almost silent. The MacPro's cooling fans and drives are louder. The original ATI 1900 (I think that's the model number) was very loud when the cooling fan was running.
It runs great even though it is now officially 5 years old as of this past April. It is the first MP 8-core that my local independent Apple store delivered.
I would be interested in a new MacPro for the promise of Thunderbolt speed, but the peripherals are not readily available yet anyway, so no hurry.
A smaller form factor, if possible, for the new MacPro would be nice.