For people having instability issues, try leaving the VRAM at stock levels and just overclock the GPU. Once you find the max GPU clock on stock voltages, start upping the volts by 0.025v increments and up the clocks again. This will help you identify the best balance between clock speed, heat, and noise.
Once you have obtained your max/ideal GPU speed, start upping the memory.
My card clocked as follows.
*Max stable core was 1275MHx with memory at 5800MHz.
*Max stable memory was 6000MHz with core at 1150MHz
*Max for benching was 1325MHz / 6000MHz but this is VERY FAR from stable and needed 100% fan.
*At stock core 860MHz, memory was stable at 6200MHz.
*24/7 clocks were set at = 1200MHz / 5800MHz.
Max stable core and max stable memory would not co-exist on my card. I had to drop one to keep the other. Also, over-volt protection would cut in if I set clocks and voltage too high. This dramatically reduced fps.
Core clocks provide greater gains than memory clocks (atleash within Heaven, 3DMark11 & BF3).
1150MHz / 5600 MHz @ 1.150v should be safe for most cards.