I have a heavily modified case and I keep a 120MM fan blowing into the rear of the stock heatsink/fan. As for overclocking, first raise the VGPU to about 1.35 or so and turn up the clocks. You will only get up to stock clocks at best (625/700) with stock VGPU. I run 1.4V for 650/775 and 1.45V for 705/805. I am getting a stronger fan soon and am going to push for 725+ on the core very soon. You will almost definitely need to mod your case for better airflow. I have at least 100 hours into modding my case. I am a chepass so I bought an old Aopen server case to modify for my needs. Paid $10 for it.
Before:
After:
Either buy a nice expensive case and get some good airflow, or mod your present case.