You've been given some really good info here. Asr33h has already stated, in order for you to see which choke/shell combination will be the best you'll have to put the time in and buy a few boxes of shells and probably even a couple of chokes. Trust me, turkey hunting is not cheap by any means, with most of the cost being involved before you even step into the turkey woods.

Do yourself a favor and go over to the NWTF websitehttp://www.nwtf.organd snoop around there and see what kind of info you come up with. the Mossberg is a very popular shotgun with turkey hunters so you shouldn't have a problem finding a few choke/shell combinations to get you started.