If your happy with the sound then just get some Berk high flow cats, saves the cash on a complete system and gives a growl and a roar at around 3k revs. Then if you decide you want to add a system after that then look into the multitude of choices available - that would be my advice.
Common upgrades for more noise are HFCs/ Intake/ Exhaust, majority of members choose to do these. Both good customers above chose an exhaust and HFCs for their set ups and both sound great.
BTW Chan, your anti roll bars arrived today, gimme a shout as and when your ready bud.
Chris