well I'm sure it's possible, but will probably void any warranty you might have and will also probably deed any tech support you might need with it useless due to the void warranty.
Other than that I've heard that PS3 uses a regular 2.5" HDD, so that should be easy enough. As for ram, if you can open it up and check what it's got, you might be able to add more, given there's extra slots available (unlikely though), so only option is replacing the original modules, which then comes down to, will the thing work if you give it any other type of memory chip other than what it's used to? I hear games are typically coded with direct I/O to hardware, so you might put it better ram, but you might not see an increase in performance if the game/PS3 is not coded to use the extra RAM.
just my thoughts, I ofcourse could be wrong, since I don't own any consoles :P