From what I have observed, ever since the fielding update (
www.whatifsports.com/forums/Posts.aspx), a bad 1B has a pretty significant negative impact on a team's defense and pitching statistics and a quality 1B can have an equally positive effect.
Unfortunately Ronnie Snow belongs in the AL and his effect on the defense probably makes his net effect like that of an average first baseman, at best. Thankfully, he would be a beast at DH and is one of the few DH only guys that should still have good trade value.