Combining two PCs

Parts that can easily be swapped between computers:
- Hard drives
- DVD drives
- Power supplies
- Cases
- PCI cards
- External devices (monitors, printers, mice, keyboards)

Parts that can be swapped, but have limitations:
- CPU & Heatsink/fan
- RAM
- Video cards
- Operating Systems
- Whole motherboards


Parts that cannot be swapped:
- Anything that's soldered onto the motherboard (chipsets, slots)

This, this.
However, I'm sure that if you REALLY REALLY REALLY wanted to, you can swap some things that are soldered on if you have basic soldering skills (at your own risk, very high risk too), extremely unrecommended and impractical. Though things like chipsets tend to use the BGA package and those are not solder-able by hand (if they were, I'd solder my Cypress chip in the display case to a PCB, and then I'll have a 5870 D=).