I see many comments here about whether waiting for Nehalem is a good thing or not. Look, an 8 core machine will be plenty fast for the foreseeable future. Besides, hard drives are the limiting factor. A year or two down the line, just plug in the latest solid state terabyte card and you are good to go.
Another thing to think about is how long it takes software to catch up with hardware. How long will it take for software to commonly use all 8 cores and utilize all the SSE4 instruction set.
The point is that your new Mac pro may get faster in the next year. Do you really want to wait 2 more years until the software catches up with the hardware upgrades for Nehalem?