As Shane said, it's physically possible to take the drive out of one PS3 and into another but it won't preserve all your data. Use the transfer utility, it works quite well and if you've only got a few things then it won't take too long. Another way, depending on what you want to save, is to backup your saved games etc to a USB hard drive and then copy them to the new PS3. You can't copy everything, but by-and-large you can copy everything you can't replace by downloading again.
I had a problem with my PS3 40Gb overheating and going into turbo recently. Found it was pretty clogged up and that having it on its side rather than laying it flat helped significantly. I also had a problem where I killed the power using the hard switch before I had turned the console off to standby (I know, don't ask) which caused it to freeze just like you describe. I found it was the HDD and it required a format of the drive (which lost me my saves, so back them up if you can) but fixed it straight away. The only thing I can think of is something got corrupted on the working space of the drive. Worth a shot if it's not just overheating.
I installed Leopard onto a G4 Sawtooth 400MHz and it ran acceptably with Spotlight turned off. So your dual 800 should be just fine. But as was said before dump as much RAM in there as you can because it'll really help general multitasking in Leopard especially.
I took apart a 5th Gen iPod with Video 80Gb to replace a cracked screen. Wasn't too had just make sure you get one of those plastic prying things to get the metal back off. Don't use a screw driver because you'll end up chewing up the plastic fascia pretty bad and it might not fit back on. For reference, you get 20hrs playback if you don't change the song because the hdd spins up only once to load the song into the flash buffer. I only ever got about 9-10 hours of real use out of it.