Most Pro and Air laptops are not user servicable. This means that nothing can be upgraded inside them. If you are lucky, you MIGHT be able to upgrade the HDD, but that is about it. Memory, GPU, CPU, its all hardwired to the logicboard.
For information on the minimum system specs for Mountain Lion, see here: http://www.apple.com/osx/specs/
The thread opener refers to the Mac Pro not a laptop.
These are Very upgradeable, you can change the processors,cd drives ram, graphic cards you can add USB3 you can raid and they have 4 HDDs that can be swapped out.
Now to the problem with software Mac Pro 1.1 and 2.1 (2006-2007) models have EFI32 firmware can not boot a 64 bit kernel however CAN run 64 bit processes.
The latest OS that runs natively is 10.7 Lion.
Osx 10.8 is full 64 bit including the kernel and can not be loaded onto these machines natively.
(You can force 10.8 to run in virtualisation or with a complex hack ! )
i am very surprised that your X1900XT graphic card still runs. You would benefit from upgrading this to one of these
It will run quieter and cooler.
I would also upgrade your RAM to at least 8GB
This graphic card and extra RAM should allow you to run Final Cut Pro X satisfactorily.
If you want to do some DIY you could also upgrade to the 2 processors but you will need to do some reading on the interwebs before you try.
Also do a search on these forums for The Hatter he very knowledgeable about these machines.
How To Install and Remove Memory Mac Pro2x2GB FBDIMM DDR2 667MHz @ $36 - These are what I have in mine and number of others, excellent quality and price 8x2GB $140
SSD: Samsung 840 128GB
ATI 5770 $244
There is a page of system requirements and what graphic card are and are not supported by FCP-X.
Final Cut Pro X, Motion 5, Compressor 4: Graphics card compatibility
WD Black 2TB $164