http://discussions.apple.com/messageview.jspa?messageID=12296219&stqc=true
MHz matter, and so too does RAM.
This article, and then browse how to choose Mac Pro, benchmarks etc on site:
http://macperformanceguide.com/blog/2010/20100905_HallofFameShame--macpro.html
http://macperformanceguide.com/Reviews-MacProWestmere-CoresExplained.html
There are always good buys from Apple Specials, from $2100 and up, just be careful or totally avoid any of the 8-core / dual processor models.
http://store.apple.com/us/browse/home/specialdeals/mac/mac_pro