I once fried a 7900GT back in the day. The days that GPUs didn't have any throttling or safety mechanisms when overclocking is long gone. The BIOS of the card will downclock automatically to not have it die. Overclocking without increasing voltage is safe enough, it might alter the lifespan slightly, but it'll not kill that card faster than the time it takes to become irrelevant.
Do note that if you DO want to look around in the BIOS, there's a good chance you can undervolt. I run my R9 270 (955MHz stock) at 1100MHz, while undervolted, I took off 5 degrees and gained 145MHz.
Tip though: leave memory alone. You don't gain much by overclocking it, it's often finicky, and 90% of the games you'll play won't see any gains.