If you get artifacts at the STOCK frequencies that it shipped at then you can RMA the card.
If you have OVERCLOCKED it yourself however that is not covered.
What "values" are recommended?
Whatever stops the artifacts.
Not all games will produce artifacts either so just because you think you're safe doesn't mean you are. I recommend using a benchmark utility such as the one Metro 2033/LL has (start from main folder), 3DMARK series, or similar, however do NOT use something like Furmark that isn't realistic towards normal game usage.
The MEMORY is likely the biggest culprit since it's artifacts and not just crashing. That means the data in your VRAM is likely corrupted.
Thus, I would put your GPU frequency back to the stock setting and leave VRAM at 6300MHz then:
1) Run a benchmark in a loop for at least 15 minutes to stress the card
2) If stable up VRAM by 100MHz
3) Repeat until you have issues then use the last stable value