Tons of information on wheel and tire sizing throughout the forum... all you gotta do is look. Use the search engine on the top bar of the page and you'll find everything you're looking for. This is the thread you need to read http://www.ramforumz.com/showthread.php?t=139106
Typically the widest you can go on the stock rims without spacers is 285's. There are people who have gotten away with wider, but the majority of people rub. If you go with aftermarket rims you will need to get the correct width and offset and then you can run up to a 12.5" wide tire.