Again, my experience has been that when i have installed incompatible components, the system just wont boot.
I havent seen any instances where components have caught on fire, or melted. I would tend to think that the worst case scenario, you will short out something on the motherboard.
Im sure you have seen posts/questions by people saying that their computer was fine, they powered it on, then heard some popping, sparks, etc... and the computer no longer boots. This would be caused by a power problem, faulty equipment, etc..
If you have some equipment that you found and you dont know if it works, i'd consider not plugging into a computer system that I've invested in. If its in a box or a drawer, chances are that something may be wrong with it, or its legacy and not worth using any longer.
Another potential issue with hardware changes is you may need to reactivate Windows. Typically this only happens with significant changes like a motherboard replacement, but it's not unheard of for minor hardware changes or even non-hardware changes to trigger a reactivation notice.