Canisters are great for both mechanical and biological filtration. You can pretty much put what you want in there, but the general idea is to have your coarse mechanical media on the bottom (i.e. where the water enters), then the biological media (ceramic rings, etc.) above that, then your finer mechanical media on top for finer polishing of the water before it's returned to the tank. You generally don't need carbon after the tank has been established for a while unless you are removing medications, etc. On a 240 gallon tank, you may consider a sump setup over canisters. I would want at least three larger canisters on a tank that size, but you could get the same results by switching to a sump for roughly the same money or less.
Big country, I think the two canisters you have on your 100 is fine and a nice setup for that size tank. I'm a big fan of the XP3 and the 350 and it's not over-filtered by any means.
