All depends on how much you want to spend. My first step would be to get a decent exhaust and extractors on it, that will give you a decent power increase plus make it sound much better. Then I would probably be a set of underdrive pullies, $ for kw you can't beat them. After that it gets a bit expensive, intake, heads, cam etc etc.
Also a great and cheap mod for the Auto box is a 'shift kit'. You can get the electonic ones from http://www.shiftkits.com.au/
or they are pretty straight forward to build yourself if you want. My preferred method though is to replace the S5 solenoid with an adjustable unit, this is far more expensive but it does offer a better shift IMO.