The worst that can happen is your GPU wont be stable and it will crash under load, it wont damage the card though. Increasing volts will make your card stable at the expense of more heat to the GPU and VRM. A small increase in voltage for a large gain in mhz is completely worth it if you ask me..
As far as temps go, with decent cooling your GPU temp should be fine. The VRMs are what I would worry about [I had in the past 2 nvidia cards bite the dust due to vrm issues..]. On my setup now with a aftermarket gpu cooler [icy vision A], my load GPU temps peak at 80c but my vrms have hit at times 100c. With a AMD gpu, the vrms can tolerate up to 120c but id try to keep it at 90-100c max.