Your temperatures are great, GPUs are able to withstand temps of over 80C so you have a lot of headroom to overclock as you only reach 51C on kombustor. Overclocking your GPU won't really make it heat much more anyway. Just bump up your core clock by around 25 each time on Afterburner and run Kombustor for a while to check temps and make sure it is stable. Keep doing this until you start getting artifacts on your screen when you run Kombustor or your GPU temp gets too high (which I doubt it will - around 75-80 ish). Once you have reached a stable core clock repeat the same process with your memory clock to achieve the highest possible overclock. Don't worry if at anytime your screen goes black and your graphic drivers fail, they will just restart themselves again in a few seconds and you'll be good to go again.
What are artifacts? I ran Kombustor again today and it peaked at around 56, I have found several different results, from the lowest of 48C to the highest of 59C on an hour or so running at max. Is there a particular setting or method I should run in Kombustor to push my gpu fully to understand if there is a weak spot in it? I will try that advice, thanks!
Artifacts are little blips on the screen that are just out of place; you'll know exactly what they are if you ever get them. I'd say instead of using Kombustor, just play a heavy game for 2-3 hours. I know on my old rig after I overclocked it, most games would run fine, but ME2 and Crysis would have sudden freezes and crashes after the card ran too long.
I'm not too knowledgeable since I've only really overclocked one of my cards but I would think the reason it freezes and crashes is because it knows staying on at the specs you have it at will hurt the card; so yea I think freezing and crashing is a safeguard against permanent damage.
Usually overclocking from stock to stable overclock gives you at most 5-8 fps depending on what it is you're benchmarking with. Of course if you're playing an older game with uncapped fps (turning the vsync option off) like CS Source it might be jumps of 40-50 fps? Maybe even bigger.