Most people around here have 15x8 rims with 33x12.5 or 35x12.5's. The 15x10's will jsut 'bubble' your tires and make them float somewhat better on the mud, but 15x8's are probably easier to handle.

As for gearing, I don't know of anyone in the area with lower gears. I ran 35's on stock gearing for a little while, and they were fine. I switched to 33's though and have noticed a little more power.

http://www.scerbfab.com/carb_faq.html