PC parts

[MetalMilitia]

Well if one is slighly less shitty, perhaps it is using faster or different memory (SDRAM/DDR), if this is the case you could not put both into a single system.
Well, RAM of different speeds may work so long as it's speed is decreased to the same as the slower ram, but generally speaking they will not play nicely together.

As for hard drives you will have to see if you have a spare IDE channel (thing the ribbon cable plugs into).