Edwin:
Before purchasing a new MacPro, I'd consider waiting until after te MacWorld Expo on January 15. It is extremely likely that Apple will announce an upgraded line of MacPro's at this conference. It is also likely that these new machines will have the new Intel "Penryn" processors (and motherboards designed for these processors) - - processors that Intel claims are significantly faster than the processors that inhabit the current generation of MacPros. What this means is that, if you wait, you can either buy one of the new machines or that you will likely be able to purchase one of the current generation of machines at a significant discount after the new machines are released.
CPU power is extremely important so getting the fastest machine you can afford is highly desirable - - and will extend the useful life of the machine.
One extremely important factor that I have mentioned elsewhere on these forums is DO NOT PURCHASE RAM from Apple. Apple charges incredibly exorbitant prices - - far in excess of the market prices for RAM. Nonetheless Apple does not manufacture RAM, it is simply a reseller of RAM made by various of the few companies that actually manufacture it (most commonly Apple sells Samsung RAM).
To get an idea of how exorbitant Apple's prices are I've prepared the following chart showing Apple's prices and the prices of one reputable third party vendor, Other World Computing, (http://www.macsales.com/) for similar RAM configurations (please note that I am using OWC as an example, it is not the only 3rd party vendor with prices like these):
Apple Other World Computing
4GB (4 X 1 GB) $699 4GB (4 X 1GB) $230 (Apple Certified)/ $198 (Apple Qualified)
8GB (8 X 1 GB) $1699 8GB (8 X 1GB) $450 (Apple Certified) / ($396 (Apple Qualified)
8 GB (4 X 2GB) $2099 8GB (4 X 2GB) $580) (Apple Certified / $320 (Apple Qualified)
16 GB (8 X 2GB)$4499 16GB ((8 X 2GB) $1130 (Apple Certified) / $630 (Apple Qualified)
(Other World Computing defines the difference between "Apple Certified" and "Apple Qualified" as follows:
Apple Certifed:
"Premium Quality, Apple Certified Modules, backed by OWC Lifetime Advance
Replacement Warranty.
Netlist supplies these same modules to Apple for use in systems factory configured
with additional memory. Equipped with Netlist's unique Apple Qualified Heat-Spreader,
these modules come off the same line as the module produced for Apple and are
subject to the additional post-production certification testing which Apple requires."
Apple Qualified:
"Premium Quality, fully tested Modules with Apple Qualified
Heat-Spreader and backed by OWC Lifetime Advance Replacement Warranty.
These premium modules fully meet and/or exceed all Apple specifications for use
in the Mac Pro Quad Xeon for a liftetime of high-performance, reliable operation.
Use of the best major brand components and a true Apple Qualified Heat-Spreader
ensure you can utilize these modules with full confidence.")
As noted above, there are other reputable vendors beside Other World Computing which I cite here because I - - and other Mac users I know - -have used their RAM for years and never had any problems. Some of these are RamJet at www.ramjet.com. Transintl at http://www.transintl.com/, etc.
Because Apple is charging up to 8 times the going price for a widely available commodity, it appears to propogandize against 3rd party vendors. Don't believe the propoganda. One of he main differences is that most 3rd party vendors offer something like Other World Computing's lifetime advance replacement warranty. This warranty means that, in the unlikely event that a chip proves defective, they will send you a replacement and not charge you unless you fail to return the defective chip within a specified period of time. Apple warranties the RAM for the same period as the rest of the computer - 1 year, unless you purchase the AppleCare extended warranty that extends the warranty for two additional years.
I hope I've been helpful,
Stephen