Yes, get a stereo amp and some bookshelf speakers. £150 will buy you something pretty decent, that will smeg all over the majority of PC speakers. That somewhat depends on if you have space for an amp. That would get you the best sound quality for your money, but some people are more interested in how much bass there is. You would need some pretty sizeable speakers to give decent bass, and even then, that may not be enough if you are a bass lover.
If you are low on desk space, or want lots of bass, then the best bet is 2.1 speakers probably. You will sacrifice sound quality, in comparison to an amp and speakers, but that cannot be helped, without going way over budget. You will get plenty of bass though, if that's your thing. AE Aego M's, are probably the best 2.1 PC speakers.