The different motherboard is likely to cause the most trouble, the are drivers for all the chips on the board. Unless the boards are very similar, you'll likely end up with a blue screen on boot. I've tried, and it's not really even possible to change the drivers out, it requires a re-install of Windows (installing over top, with the repair function will probably get you a running machine, but it'll have a lot of left over junk).
Swapping video cards is almost as bad. Most other peripherals can be changed.
