Nehalem is an amazing CPU and plays with PT very nicely. Check out this thread for benchmark results:
http://duc.digidesign.com/showthread.php?t=238426
Things you might want to consider about Mac Pro:
- If I recall correctly, PT cannot see all 16 cores ( (2x4 core CPU)x2 with HyperThreading)
- Current Mac Pro line is about to be refreshed. Hold on a few weeks for a 'free' upgrade
- Currently shipping Macs have Snow Leopard (OSX 10.6) preinstalled. So far Pro Tools does not play nice with this OS.