Advice on surround sound system

I was wondering if people could give some advice on surround systems. I currently have a 5.1 set for which I paid about £200. I want something better, I think I would be willing to spend another £400. What's best to do?

Is it better to just replace the speakers I have for the current system? Or should I just buy a new set? Or is there hardly any difference in speakers considering I'm 'only' willing to spend £400 on them?

Any words of advice are appreciated!

(I got some info from people on Google+ as well, but thought there might be more experts here?)