Patti,
I too am new to the text messaging vs email, data cost world. I have Cingular so telephoned their customer service reps for cost, best methods, etc. One advised me it was better to use the email for text messaging to save money. If I remember their data packages start at 5mb, on up to unlimited for $39.95/mo.
I was very confused with what I needed for my purposes. I called them more than once until I was confident I understood the differences. The rep at the store sold me the wrong data package. At Cingular's network, the requirement for the TREO is different than for the other types of smartphones or cellphones.
That's what they're there for - use 'em. You're paying for the service in your total bill anyway!
Bob