These temps seem about normal, but I wouldn't advice overclocking that graphics card much more using stock cooling. The temps may vary depending on the actual load on the graphics card. Some games are more graphic card intense then others so you may see some different temp readings as well. To stay on the safe side it's good to consider upgrading cooling as well.
EDIT: if you see artefacts appear then that's usually is a sign your memory clock is set too high.