If you ever have a problem and need to reinstall, you will have to purchase new disks. Apple will sell you new copies of the original restore disks or you can buy a new copy of snow leopard and a new copy of iLife.
Snow leopard is 29$ ( althought to be 100% legal you should be buying the 99$ pack (iLife included) I am not sure about the legal stuff, if you own any software licenses that are on a used computer or not (on windows it comes and goes with the serial, but on a mac...)
well you should be fine with the $29 "upgrade" license , which also works for reinstalling