I am looking at some Boss rims that are 20x8.5, with the stocks being 20x9(supposedly). Will it make a huge difference looks/performance wise? What tire width can I go to?

That wheel sizes will be OK just get the right offset on the wheel, as for your tire size it all depends on what u want your truck to look like, GOOGLE the BOSS WHEEL with the size of tire you might like on a DODGE RAM CLICK IMAGES try 305/50/20 a 33" tire should ge OK as the stock tire is 33" GOOD LUCK