Focal CMS 65s, or 50s if your desk isn't that big (I'm getting the 50s), there are Genelecs in that price range, Adam A5X or A7X. You shouldn't buy anything without auditioning it first, as different speakers suit different tastes, and only your ears can decide what you like best. I see you are in London. I thoroughly recommend going to West End DJ's production store, which is in Great Russell Street, just off Tottenham Court Road. They have a bunch of stores in that area, but the production one is where they have a basement set up where you can audition a bunch of speakers against a huge digital library of music. I listened to the Focals there, although they have Adams and Genelecs too, as well as some other stuff. It's cheaper to buy speakers online, but they are nice people, letting you take as long as you need to switch between different speakers on the fly, and if you have any problems then it's not hard to take it back. (P.S if you buy the Focals remember to take the woofer grille off and put the tweeter cap on!)
If you're getting a speaker of that quality though, a few things to consider:
Desk isolation, placement and room treatment can all affect the sound noticeably. Do what you can to get the best out of the speaker, read up on these things.
Source is important. Changing speakers will always provide the biggest change in the audio chain, but they also deserve to be fed from a good source. Even a fairly budget external DAC is going to take things up a notch over any onboard sound, or most soundcards. You won't need an amplifier with these speakers, as it's built in, although you will probably want something like a TC Electronics Level Pilot or a pre-amp to control the volume of the speakers, as they have separate controls.
Badly recorded/mixed/mastered stuff will sound bad. Well recorded/mixed/mastered stuff will sound awesome.
My suggestions are probably complete overkill if you're not addicted to music and high quality audio.
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