Make sure you set the "Power Control" setting in AMD Catalyst to +20%.  If I leave mine at 0, I get a speed increase up to 900 MHz, but then the speed goes down.  With the "Power Control" setting at +20%, I can overclock to 950 with 70 degree temps and 420 Mhps using the bitminter client.  Check out this thread:

http://forum.bitcoin.org/index.php?topic=22432.0