Unless you have a custom water loop to cool the GPU, you shouldn't overclock beyond 20% of the GPU's standard clock speed; so 1200Mhz should be the limit for a 1000Mhz standard clock. Side intake fans will help, but only up to a point.
In theory, if you ensure that the GPU has sufficient power and you keep the temperature low, you can overclock it as much as you like. In reality, your motherboard probably won't take the strain and games will freeze or BSOD.