It looks like the upcoming GeForce 8800 GT will be very overclockable:
Nvidia claims that there is a lot of space to overclock both the GPU and Shaders and it looks that 1,500MHz for Shaders is just the beginning. Many of the usual suspects will introduce their overclocked cards as this is the only way to differentiate yourself from the others.
We don’t know the limit but Nvidia endorses many to overclock the Shaders and it even plans to support the Shader overclocking in the next version of Ntune.
You'll be able to buy the card in retail stores on Monday, or perhaps even a bit earlier if you're lucky.