Which ram size is best for a pentium cpu of 3 0ghz

Hi. As much as you can afford to get. A minimum of 1gig for Vista is needed really. If you are running Windows 32bit , You can only use a maximum of 4gig as windows will not recognise more than this amount.

http://www.crucial.com/systemscanner/

Download and run this it will tell you what ram modules you need.