Certainly a well calibrated wide-gamut NEC display will help you with critical color work more than any iMac will (or any Apple display, for that matter). Apple has promised a new, upgraded Mac Pro for 2013. When it will appear and what it will have is anyone’s guess, though there’s reason to believe it will offer thunderbolt and/or USB 3.0 port(s). If you can wait (...and have the money!), this - combined with an NEC display - would definitely offer you the best possible hardware.
Another alternative would be to upgrade the present Mac Pro with, for example, a fast SSD (mine rests in the lower optical drive bay of my 2009 Mac Pro), a USB 3.0 PCIe card (can warmly recommend those from CalDigit), increased memory, and perhaps a RAID 0 (Stripe) system utilizing two of the four internal hard drive bays. This would certainly offer you more than enough performance for any photo processing you need to do.
Best of luck,
Larry