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,