Which card was installed? Can you not just replace like-for-like?
Best way forward would be a HD4650 or if you want more zip the HD4670. Neither needs an extra power connector and neither draws more than 48Watts at full load.
Check: Overclockers, Scan, CCL, E-buyer and Eclipse online. Expect to pay between
60 and 70 pounds and do n`t get the more expensive 1Gb cards.