what to do xp or vista????(pls ans fast!!!)?

ok now i have a qutri dio 2GB RAM(DDR2) 512MB NVIDIA 9Series and of abt 3Gz with a 160GB harddrive it will be used by my grandfather for word PowerPoint and excel and by me for internet & gaming.So which is better xp or vista.oh and my grandfather has used comps for abt 9years.
basically i want to kno if it would be difficult for him to use