Search - 'AND' ... 'OR'

Hey all, I am in the middle of writing a script to allow users to search through news archives on my website. Anyhow I am just wondering which logic you believe best suits a search. i.e. say a user types in 'games AND playstation' then only results containing both those words will be returned by the same token they could type in 'games OR playstation' then results containing either word will be returned However if a user does not explicity join there search critiera i.e. 'games playstation' what should I assume? What would you expect? Google for example wacks an 'AND' inbetween which is wise considering they are searching billions of pages however my site obviously will only be searching a few hundred so maybe I should assume an 'OR'? So basically I just am curious to know what you would expect when entering multiple critera. Regards, Scott