kenwood double din head units are good as well. You can get them for around £500.
These have sat nav & bluetooth built in and if you have the steering wheel controls, they can be linked up to these and display info on a highline cluster.
I play movies and music vids right off my ipod.
I couldnt recomend these enough and there was a deffo sound improvment from my standard speakers although i have the 10 speaker upgrade
I bought mines a few years ago for my mk5 at a cost of £1100!!
Still works brilliant and is now in my new mk5 gti.
RNS units are good but personally think you get more for your money with the kenwoods.
This is a pic of mine in my old mk5