Hi Alex, first question is what resolution are your monitors?
However with your budget and the info available at present I'd recommend an HD 6950 see link http://www.overclockers.co.uk/showproduct.php?prodid=GX-030-C3&groupid=701&catid=56&subcat=1752
This will leave you a bit of money left to buy the Display Port adapter you will need for 3 monitors. I can't advise which one to get as I don't know your monitor connections.... I'm also wondering what type of games you play as I think a single 6950 over 3 monitors will struggle anything over medium settings. Let us know how you get on?
looking for BF3 and Crysis mainly 😛 and at the inuet i think im on the stadard 1200x800 (ish) at 19" but may be buying bigger monitors soon anyway so whatever res is needed i can buy.