If you want to do it cheaply get someone to cut and weld this
http://cgi.ebay.co.uk/PECO-BB2-Pipe...013981593QQcategoryZ38786QQrdZ1QQcmdZViewItem
into your exhaust system along with the other silencers.
I think the best option for you if you don't mind spending a bit of cash is
A. a cherry bomb supertune silencer
B. a supertrapp silencer
The supertrap is designed for forrestry rally stages where there is a need to suppress any flames from the exhaust, because of the way its designed you can add or take away silencer discs to achieve certain levels of tune and noise. It's quite individaul looking too, not many people have them.
The Supertune has an adjustable 'butterfly' within the exhaust that you can turn by hand to adjust noise levels. This would probably be best for you as it is really easily adjusted and you can change it for motorway or around town etc
One thing to bear in mind is that both these silencers are designed for competition / sport use so they could actually be just as loud as your current set up, therefor you may have to add an additional silencer anyway.
Unfortunately performance and noise levels don't neccessarily go hand in hand you are probably going to have to choose which is more important to you.
Check your system isn't blowing firstly though
Let me know how you get on
Amy