Usually, this information is in the specification of the card (Nvidia keep a library of that info on their product pages). If I recall correctly, it's usually somewhere in the range of 90-100°C as a stated maximum, but I wouldn't let the card get too close to that kind of range just to be safe.
Dust is usually the culprit here. Buy some compressed air, turn the PC off, take the card out and carefully spray the dust out of the heatsinks and fans onboard. Leave it a minute to be sure the propellant is all dried up, then slot it back in.
It's not a bad plan to do this frequently, I tend to clean both my graphics card and CPU about once every two months in this manner. You should notice a good drop in temperature.
As for the open side, well, that can actually harm your airflow, depending on your PC case. Keep it shut while it's running and let the fans run clean is a better bet. If your temps are still higher than you'd like, and your chassis has the vent space, new fans can be picked up off ebay for cheap - just consider where you're pushing air around - more fans fighting with each other just equals wasted energy.