Try some of the fol lowing links for ideas:
My opinion is based on what I see happening in my daughter's and son's homes --
My daughter's two sons play all kinds of video games, including some of the most violent out there. Neither is a violent boy, and they are not becoming violent -- not because of the games or any other reason. HOWEVER, the younger boy's vocabulary is not as broad as it should be by now (to him, everything is instantly related to the military in some way!), and the breadth of his thinking is stunted, in my opinion. The older boy loves to play all kinds of games, but is not addicted to any of them! He is also in university now, studying for an engineering degree.
In my son's home, his daughter plays only the games on her Nintendo DS and focuses on her reading, her studies, her artwork, etc. She is a much more "rounded" child, and her educational progress so far is amazing.
From what I've seen in my own family (including the above examples and those in other family members' homes), these games don't make my grandchildren, nieces, nephews, et al., violent, but they can have a negative effect on children's ability to think well, to focus in school, to WANT to develop a variety of interests, etc.
Thank you, your responses helped my very much.