When I was in grad school, I had an opportunity to work in a lab that was examining MRI brain scans to compare brains of "normal" people versus those with OCD and Tourette's Syndrome. In particular, we were looking at the ratio of brain matter to ventricles, the fluid-filled, non-brain matter areas. Interestingly, the most abnormal brain in our database belonged to the Principal Investigator of the study who was a really nice, very sociable, very smart man. It seemed that gray and white matter don't necessarily matter.
Apparently a team of doctors at the Hopital de la Timone in Marseille came across a man who was admitted for mild weakness in one of his legs, and upon scanning his brain, found that there was extremely little brain matter -- just mostly ventricles! Although he had a slightly-below-average IQ, he had been living a pretty normal life up until then.
The article explained that he had hydrocephalus (essentially, extra water in his brain) when he was a child, and had undergone an operation to drain it, but hadn't had any problems since then. Research indicates that this condition and procedure do not necessarily lead to low IQ or other problems later in life.