Yes you can overdose on water. It is called water intoxication. Behaviors of drinking too much water and water intoxication resembe alcohol intoxication. Swelling of brain tissues can cause seizures, coma and ultimately death.

You can read about it here. http://chemistry.about.com/cs/5/f/blwaterintox.htm

Healthy adults require approximately three quarts of fluid each day. About half of this comes from food, with the remainder from beverages, So the advice to drink eight cups of liquid each day is good advice. If you exercise, are breastfeeding, are ill, or live in a hot climate, your fluid needs are higher than the three quarts each day - Eight cups each day.