if saving money is your goal, and u have a previous windows version cd, get the upgrade, the only diference is that it will ask u for the previous version cd, just to verify that u have it, that is only diference between upgrade and full version. i would also recomend xp pro. and would create a slipstream install cd with sp1 or sp2, to make it all easier.
i personally only use sp1, cause sp2 causes me problems.