As far as I know 2" each side in the rear is about as much as you'd want to go. If you really wanted to you could run 2" + g-force hubs for a total of close to 8" but you'd be as wide as a Hummer and I don't think it would stand to any abuse. I've been at 2" each side for 2 years now and it's a good setup, no issues.
As far as the front goes, you don't want to use spacers but if you have to, keep them as short as possible. If you need more than 1" on each side, I'd go with a more elegant solution (either get some wider arms or a diamondj kit).