The width of the spacer you can use is going to be somewhat dictated by the backspacing of your rims. You could use a 1.5" spacer, then go with a 5.5" backspaced rim. That will push the rim out a 1/2", but give you more of a flat front look to your rims (more like what the 04+ vehicles look like).
You can go wider, but also keep in mind that by pushing the wheels out further, you are going ot limit how big of a tire you can run for a given amount of lift since the tire will now be moving in a bigger arc. Also, atleast on the front end, you can anticipate more hub wear since you will now be trying to twist the hub sideways vice keeping the center of the tire in line with the center of the hub. I know just stepping up to the 37x12.5" tires, which moved the center of the tire out about 1", the combination of the taller tire and slight off center has resulted in me replacing hubs about every 30-40K miles.
Just giving you first hand experience.
