Most people who overclock the GTX series use the stock cooler because it does a very good job. I can't tell you the highest frequencies because they vary from card to card, but your can normally put a 100MHz overclock on your core without issues.
I have the exact same GPU as you with stock cooler also. I use EVGA Precision X to overclock my GPU. The default **** for it is 648mhz gpu clock and 1242MHz shader clock. I have overclocked to 710MHz GPU clock and 1365MHz shader clock without voltage overclock and have great results. My fan spedd is at 75% while gaming and never goes above 65c. This GPU can get upto 75c before you should worry about it getting too hot. Even though it can handle around 85c 75 is probably as high as you want it to go. Hope i helped.