"Major" upgrades are paid upgrades. To go from 10.1 to 10.2, or from 10.4 to 10.5, or from 10.5 to 10.6, you must pay and purchase the new version.
"Point" upgrades are free. To go from 10.5.4 to 10.5.5, or from 10.6.1 to 10.6.4, you simply run "Software Update."
For future reference, Apple posts all the operating system and hardware requirements for their hardware (and software) clearly on the physical box that the item is contained in as well as on the product's page on their website so that you know exactly whether or not you can utilize a piece of hardware before you purchase it.