Can i just add txt messaging to my cellphone plan?

How much is the cheapest & how many txt messages can i send?

It's an AT&T/Cingular plan 'Nation 550 FamilyTalk w/ Rollover Mins'

Messaging Starter - $4.99 - Any 200 messages for text, IM, picture or video.
Messaging Unlimited - $19.99 - Send as much as you want!
Family Messaging Unlimited - $29.99 - All lines in your family plan can send as much as they like!