OS X Mountain Lion will not install on my MacPro, even though it will load to my MacBook Pro

I have started this thread on a different heading, but I thought that I'd just ask under a slightly more specific headline, to maybe attract attention from other users who may not have already seen the other posting. And also in the hope that perhaps there may be a super boffin out there who may be able to assist.

Basically, I have bought, and successfully installed OSX ML 10.2.8 to my MacBook Pro [3,1] - bought Feb 2008.

Saved the installer to load to my MacPro, which I still needed take up to 10.6.8, before doing the 10.8.2 update.

Did the 10.6.8 update, no problem, from 10.5.8.

Updated all the "software updates" as required, before the 10.2.8 could be attempted.

Tried to do the 10.2.8 level, only to then find that the error message thrown back at me:

We could not complete your update.

OS X ML is not compatible with this computer.

OK, so now after a lengthy discovery, I understand that it is most likely as a result of the "Model ID" which on my MacPro is [2,1], because it was bought late 2007 [September 2007]. Even though, when I bought it as the highest spec build to order option that was available at that time:

MacPro2,1; Quad-Core Intel Xeon; Dual Processors of 3.0 GHz — i.e. 8 Core, 3.0 GHz

Bus Speed 1,33 GHz,

Fitted the highest spec graphics card [at that time] — ATI Radeon X1900 XT [512MB],

Running 2 x 26 inch Samsung SyncMaster T260 [connected to the two DVI connections the ATI card has].

And it has 24GB ram [4x2GB and 4x4GB] — 667 MHz, DDR2 FB-DIMM

Even added the Bluetooth module, as in here in South Africa, that was not sold as "standard" and needed to be added, at additional cost.

By comparison, my MacBook Pro, whilst admittedly is just slightly not as old, bought in Feb 2008. It is certainly is nowhere nearly as "equipped" as the MacPro.

See for yourself:

MacBookPro3,1; Intel Core 2 Duo; 2,4 GHz;

17 inch; Hi-Def; 1920 x 1200; matte screen, brushed aluminum body with the silver metal keys — NOT the uni-body.

Fitted the max ram that was reported to be supported, at that time, 4GB, 2x2GB — 667 MHz, DDR2 SDRAM

As I opted for the higher definition monitor, graphics card on the MacBook Pro is the NVIDIA GeForce 8600 GT 256 MB

Naturally, as it was a portable it came standard with the connectivity that they do; Wi-Fi / Bluetooth etc.

The MacPro, as to be expected, has far more USB, FireWire, and network connect ports compared to the portable, but there was no thunder bolt nor USB3, at that stage, so that can't be a factor here, as neither of the Mac have any of those two technologies onboard. So what the blazers is the reason that my MacPro, as it stands, is supposedly not as capable as the MacBook Pro?

Or what else is the reason that 10.2.8 will install to the MacBook Pro, but not my MacPro?