High sodium diets can cause hypernatremia. Studies have shown that too much sodium can put some people at risk for high blood pressure (hypertension) which often leads to strokes or heart disease.
That being said, hyponatremia (too little sodium) can also cause health problems. Symptoms include bloating and puffiness in the face and fingers, nausea, vomiting, muscle weakness, headache and disorientation. Too little sodium can cause cells to malfunction, and extremely low sodium can be fatal.
Nobody really has to worry about hyponatremia though unless you run A LOT. Reason is is because running causes you to loose this electrolyte (through your sweat) and with the consumption of a lot of water, you can serverely deplete your bodies electrolyte balance which can lead to hyponatremia.
Having adiquate sodium in the body also helps with the conduction of nerve inpulses and muscle contrations.
After cardio to replace the lost electrolytes. Other than after cardio, just eat throughout the day. You will realize that you can easily reach 3,000mg a day with a normal diet.
I've read that around 3,000mg a day will suffice.