Ok, thanks. In the PC-world you could just change the motherboard, cpu and ram. Is that not possible on a 3.1?? I am such a noobie, but i have falling in love with Mac.......(finally)
No, the 3,1s used the Penryn-based Xeons and the 4,1s/5,1s used the Nehalem and Westmere based Xeons. When they switched processors, Apple redid the layout of the logic board (with the 4,1/5,1 it's really a backplane board). Instead of the processor(s) being mounted directly to the logic board and the RAM being on two risers cards, they moved the processor and RAM to an integrated board which sits as the bottom of the system case and uses a connector for power and data.
With this change, Apple had to completely redo the internal layout of the case. The only parts that I know are interchangeable between the machines are the CD-ROM caddy and side panel (although the RAM replacement instructions on the door would be for the incorrect model).
Now, the firmware can be upgraded in the 4,1 to 5,1 so the user can take advantage of the Westmere CPUs (6/12-core options are the main reason) and faster RAM (1066 vs 1333MHz as long as the CPU supports it).
Basically, the highest amount of cores you can have in a 2008 Mac Pro is 8. In a single processor, non-upgraded 2009 Mac Pro it's 4 cores while in a dual processor machine it's 8 cores. Upgraded 2009 and 2010 single processors can support up to 6-cores, while the dual processor models can support 12 cores.
