The only internal components that can be transferred are SATA hard drives, IDE optical to a 2006-2008 Mac Pro, and SATA optical drives to 2009 and later Mac Pros.
Of course, drives will need to be reformatted to GUID partition map to be bootable.
Reading from APM formatted drives is fine.
RAM and graphics cards are definitely out.
PCIe expansion cards are on a case by case basis. Some will transfer, others won't.