Just kick the exhaust off. It'll sound like a WRC.
You can also get really cool whistles that you stick into your exhaust that are supposed to sound like a turbo. They are really good at annoying the crap out of anyone around you. Every time your car is running it sounds like a footy referee on steroids
Seriously though. Keep it clean and running smooth and invest any spare change in actually inviting women out, works way better than a fat exhaust or rims
A nice radio and a decent set of speakers running through an amp can be nice though, as long as you don't try and be heard from a mile off.