Look at the intense selection on the Ashkenazi Jews over maybe 1500 years which has only produced an average IQ of 115 (there are alternate theories for their high IQs).
http://en.wikipedia.org/wiki/Ashkenazi_Jewish_intelligence
Selection of dog breeds for intelligence (German Shepherds, Jack Russell terriers, Poodles vs eg King Charles Cavaliers, Corgis) seems to have produced a large gap but it took a long time - dogs have been domesticated for ~5,000 years.
Generally selection works quickly at first, by filtering the population for the desired trait. Then it slows dramatically as the process is limited by new beneficial mutations which are rare.
It is noteworthy that genes for high IQ seem to come at a price. Read up about Einstein's son Eduard for example.
Final point: maybe parents will not want to select for IQ. Maybe they would prefer to select for beautiful daughters for example?