There's a lot of questions like Rooz suggested before one could proceed further on having the changes applied to the in car audio system.
By default system I would imagine u're looking to change the:
*Speakers (if you're looking for enhanced sound quality), however this isn't an important change
Which is something to what i'm looking to get done soon. Try looking at bundle packages like what Halfords is offering. Subs + Amps. They offer half price store fitting too for some of them. They got decent subs + amp packages, all you gotta add is a good head unit and you're ready to go.
As for the speakers, BMW install decent speakers (yeah ok they're not awesome, but they're good enough) so you may not need to have that changed
Personally i'm going for an ALL Alpine system, everything Alpine!
Yeah the cost will touch £900 but gotta put in a SLICK system!
By the way, I hear the amps that are pre-installed in most BMW's aren't that good, at least not compared to the speakers (which are quite good) hence why I didn't recommend changing speakers over an amp.