A lot also depends on how far you want to go.
I got a 3.5 pulley and got a bigger tensioner pulley as well. By doing so, I didn't need a smaller belt. Also, with the 3.5 you don't need to do anything else, unless you want to. Outside of my smaller 3.5 pulley, all I did was switch my stock ait filter with a K&N filter which causes me to breathe better. The K&N is the same shape and fit of the stock filter but breathes around 30% better.
Note that my way isn't the best high performance way, but is the cheapest since you won't need a CAI or exhaust work,and you will still gain around 25 horses and 40-50 ft. lbs of torque. Not bad at all!
ps. various places sell a 3.5 pulley. I think ZZP does. I got my tensioner pulley from them. Others can chime in on the places who sell a 3.5 SC pulley.