How much are we talking dude?
Hard to say cos some of my system is a few years old now - been bringing it with me from car to car.
Alpine IPOD headunit is about £320-399
New amp im getting is £399 - its better than my old one that cost £800 ten years ago
Front componants were about £300 - you can get basic stuff for under a ton
Back mids were about £400 - again - basic stuff avaliable for under a ton.
Sub was about £400 with the box
You could buy a new head unit and use it to power front componants - would be under £500 ... then add an amp and sub at later date for another £500
To be honest anything is gonna be better than what renault have put in there ..... my 25 year old nova first car had better sound ... I can not understand why the standard unit doesnt go any louder - its not like its clipping or anything at full volume - the standard speakers could easily handle more amplification