For speakers, buy a cheap amp and speaker setup from Richer Sounds, as Derek pointed out. When it comes to PC speakers, it doesn't matter whether it claims to put out 200 watts or 1,200 watts - PMPO figures are a load of bull. The RMS figure gives the true power of an amp.
£180 spent at Richer Sounds will get you a Yamaha RXV350 AV receiver that puts out 90W RMS per channel, plus a Yamaha NSP100 six-speaker package to go with it. The sub has its own 100W amp built-in, so you've got a total of 550 watts RMS to annoy the neighbours with!
Oh, don't forget to take out the Richer Sounds Supercare warranties, they're dirt cheap and you can cash them back in if you don't use them!
Even more info on the RXV350 here and the NSP100 here.