Water intoxication actually occurs well before you can overfill the system as you suggest.
Rest of post per Wikipedia:
Body fluids contain electrolytes (particularly sodium compounds, such as sodium chloride) in concentrations that must be held within very narrow limits. If water enters the body more quickly than it can be removed, body fluids are diluted and eventually a dangerous shift in electrolyte balance occurs.
Sodium is not the only mineral that can become overdiluted from excessive water intake. Magnesium deficiency can cause metabolic changes that may contribute to heart attacks and strokes.
Consuming as little as 1.8 litres of water (0.48 gal) in a single sitting may prove fatal for a person adhering to a low-sodium diet, or 3 litres (0.79 gallons) for a person on a normal diet.
> Water intoxication actually occurs well before you can overfill the system as you suggest...
WOW. I didn't know that. Less than half-a-gallon: that's like four pint glasses!
Thanks for the insight -- I learn something new every day from the Free Republic.