If you are looking for a performance boost out of your card as far as overclocking, use MSI afterburner and Furmark to see what your card can do within your comfortable temp range. Stability issues means you need to slow the card down. every card is unique, so there are really no "perfect settings." However faster memory or core clock speeds never hurt performance as long as they are 100% stable.
If you are talking about in-game quality settings, then that is very dependent on the games you want to play. If the graphics are choppy, back them down.