Simple question, would a HD4890 run ok on an OCZ StealthXStream 2 500W Power Supply - http://www.overclockers.co.uk/showproduct.php?prodid=CA-045-OC
The system it's going into would more than likely have a Phenom II X4 925 or similar, 2 x 7200RPM HDD's, an ODD, 3 or 4 fans.