Be careful, as far as I know you can't use a current Apple Cinema Display with the Mac Pro because the Mac Pro doesn't have thunderbolt, only displayport. Someone correct me if I'm wrong, but it's something you should research.
Shouldn't matter though, the ACD is a bad choice for design/photography work anyway. Due to the extremely glossy screen it's hard to focus on tiny details especially when working with dark designs or pictures. Reflections will constantly be in the way, even in a dark room. Get a big Eizo, Nec or Quato monitor.
I'd rather invest in a really good screen before the processor upgrade. Like others have mentioned, the biggest performance boost in terms of value for money would be a ssd.
