Even though I voted yes because I enjoy OCing, really I don't think you're going to see much difference. You may get a few points on some benchmarks but in the real world it's tough to see a few extra framerate points. More often than not some other factor will make more of a difference.
In terms of risk, I don't think there's much if you have a good power supply and cooling. Just bump up slowly if you're going to do it.
Agreed. You're going to get an extra 5%-10% or so. In the real world, you're talking an extra frame or two per second. On a very, very negative side, it's going to make your computer even louder as the fan(s) on your card are going to ramp up to deal with the extra heat...