Horns can be good if you use valves or digital amps. I heard some lovely Cain & Cain a couple of weeks ago. Those were very highly resolving in the context I heard them but not for computer use.
My suggestion would be a T Amp with B&W control 1 if you're doing it on the cheap or if minted, the Fujitsu Ten Eclipse speakers that are intended for the ipod for £350-ish. They image like anything and are good for nearfield. One of the dealers sells them on Ebay. I also saw them in the Apple shop on Regent Street in London.
If you're going for serious stuff you could try www.turnkey.co.uk and look at the powered monitors. Dynaudio do some good ones.
You used to be able to get Wharfedale Diamond Active monitors. If they are still sold, that might be a good option. I saw them on sale as little as a year ago so they did develop beyond the famous Mk 1 versions.