There is no way to really pinpoint a safe voltage. However, I can tell you that you'll want to keep it under 1.4 volts. You'll need to watch a few tutorials on youtube first to find a decent range of voltages. Then test each of the voltages in prime 95 until you find stability for a few hours or so. Safe temps are usually in the 65 C and under range. Most people don't like to exceed 70 C on a load. On full load such as in prime95 you'll want to stay under 75 C at the very least. But if I were you I'd still shoot for 65-70 C.