To be honest, your current 5.1 speakers will fare well compared to a newer set. Budget PC speakers don't see a lot of change over the years, so an older set that cost £80 a few years ago, will not be much different from an £80 set now. You'd have to spend more money on better speakers to get an improvement. As there is little choice once you go past Logitech Z506, you're looking at £200 minimum, unless you can get a good used bargain.
A sound card though might improve things somewhat if you are using onboard audio. A Xonar DG/DGX would be a good cheap option.