RicS:
At some point CO2 will definitely cause cooling
Where did you read that? The effects of CO_2 can be predicted more or less from first principles. You know the properties of this molecule, its absorption spectrum. So it is relatively straightforward to calculate the effects this gas has. The cooling caused by volcanoes is due to the airosols that are blown into the stratosphere. Because there is no clouds and rain there, they stay there for a few years. CO_2 stays in the atmosphere for many hundreds of years.
Venus is actually a good example of the effects of CO_2 has as a greenhouse gas:
http://news.bbc.co.uk/1/hi/sci/tech/4335628.stm Pay special attention to this:
"Even though Venus is closer to the Sun, its cloud cover is very shiny and reflective," Fred Taylor explains.
"If you work out the energy balance, Venus is actually absorbing less heat from the Sun than the Earth is. By-and-large, you might expect surface conditions to be the same."
for another extreme example of the greenhouse effect.
