Chip,
The 2.3 IS the sweet spot in the NEW 2005 models, but if you can find a 2004 Rev B 2.0 , you get the same 8 slot/133MHz PCI-X PRO motherboard for way less.
Apple Pro applications love RAM and your best performance increase will most likely be noticed by adding that extra RAM rather than relying on just processor speed.
I'm recommending the 2.0 Rev B because of the overall value and proven performance and that fact that you'll have a fantastic machine that will hold you well into 2010 and beyond.
Since you work right next to CompUSA, you should be able to get the store
manager to drop his price on the remaining NEW in Box Rev B's he has in stock. AND you may still qualify for the $9.95 Tiger upgrade from Apple.
If he tries to go more than $1749, I'd look elsewhere.