It depends. If you are running Vista Home, you probably will do fine with 512mb. If you are trying to run Vista's Aero interface from Vista Premium or above, you will probably need at least 1gb. For an optimum experience I recommend 2gb of RAM however, it certainly doesn't stop there. If you are running graphic intensive programs or running some heavy media type applications, you may need more than that. Not only the RAM will make for smooth operation of Vista. A dual core processor helps quite a lot too.