I would say that if you are using a standard headunit then I would spend about £50-80 ish. I would also fit some sound deadening product to the door around the speakers. Makes a big difference, and at £20 ish a easy upgrade. I fitted a underseat subwoofer to my Stilo and that also changed the sound too. By far the biggest upgrade is to change the headunit to an aftermarket one. There's loads of reviews for product's so let the internet guide your purchase.
Also it's the law to post up a few photos to the forum and a small review too.