Max temp should be somewhere around 100c.As for a safe temp I would say anything below 90c is a safe temp.The 6750 should normally run in the 70c range.
With fan at 35 percent (not system-controlled) I idle at between 39 and 41 C. I normally put the fan up to 70 or 80 percent for heavy gaming...I've seen the card max out at 67C (figures from HWMonitor)