The maximum GPU temp for this card is 105c.
Generally speaking, there is no "safe" overclock, even with two identical pieces of hardware, one may be able to achieve higher clocks than another. Your best bet for overclocking, is to try bumping up the Core Clock and Memory Clock about 5-10Mhz at a time, applying the changes and then running a stress test to verify if it is stable or not. You can keep upping it a little at a time, until you start experiencing artifacts or issues, then taking it back down to what you had before without any issues and that would be your "safe" overclock.

No, you cannot "just keep upping it forever".....and yes, under 90C is a good "safe" high temp to not exceed.