What are the effects of taking more than the prescribed amount of citalopram?

Depends on amount. Taking more or less of a prescription medicatoin than the doctor has ordered is rarely if ever a good idea. Accidental missed or double doses do occur, though. With citalopram, a single double dose probably will have no noticeable effect. Very high doses of this and other ssris can cause "serotonin syndrome, " however, which includes restlessness, elevated bp, and fever and can be fatal.
Celexa (citalopram) The fda recently issued warning regarding daily doses in excess of 40mg. Due to toxicity.