I've just fitted 2 inch spacer blocks under the standard springs and so far all is well although I could do with a set of new (plus 2 inch) rear shocks as I have managed to fully extend them with a bit of a thump.
As said, there's loads of info on this subject if you do a search.
2" is pretty much the limit without changing anything else but if you're removing your ARBs then check your flexible brake hoses on full drop as you may need to extend these.
