You should have a clock ratio for GPU/MEM clock of at least 0.7, best
results with 0.57 (0.57 for example GPU 900MHz --> MEM 1,580 MHz).

Also if you are having problems with artifacts, i am sure you are using
the Catalyst 13.x Driver. Try for example 12.6 to get rid of artifacts while
mining. (worked for me at least)

Edit: Try 825 GPU and 1,450 MEM clocks at I 19. That should give you
almost exact 500KH/s