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
http://store.apple.com/us/product/MC742ZM/A/ati-radeon-hd-5770-graphics-upgrade- kit-for-mac-pro-(mid-2010)?fnode=53
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.