Mk4 nav is the best navigation system you'll get with the stock screen.
Superb, full aftermarket audio and navigation systems are possible (see thread above) but not easy.
The stock nav / screen can be used with aftermarket amps and speakers for much improved audio. The exact steps depend on whether you have a DSP amplifier fitted to your car. Loads of info available about this.
The stock amps, nav and screen can be kept with a speaker upgrade that gives a worthwhile improvement in audio with minimal fuss. Careful speaker selection is vital if keeping stock amplifier (Bavarian Soundwerks sell an often used kit that seems to work well).