Which iPods are you talking about? All the ones with a two plate design, like the classics and 1st/3rd gen nanos can be taken apart easily, with a screwdriver or putty knife, but you will want to use a plastic one to make sure that you don't scratch it. As for space upgrades, it will require another nano if you are wanting to upgrade the falsh based players. I know for a fact that the first gen nanos had space for another memory chip, but whether this is true for the newer ones I am not sure. For the hard drive based ones, it is a matter of finding a hard drive that is the same physical size. If you do some reading (quick search through Google brings some decent results) you can find some good hacks, and they are pretty easy IMO.