If you set up your e-mail account as a BlackBerry Internet Service (BIS) account, then the account is polled every 15 minutes for new messages. If you want the instant push e-mail you can configure your e-mail account to forward to a BlackBerry-specific e-mail address that your carrier provides (i.e.
).
If you want instant push with wireless reconciliation then you might want to look at a hosted BlackBerry Enterprise Server (BES) account. This way you contacts, notes, calendar and tasks will be synced, and there will be two-way, instant, wireless reconciliation of messages.
You will need to make sure the data plan you have is a BlackBerry specific data plan. I am not sure of the cost, but the plan should say BlackBerry in it somewhere. If I recall AT&T's unlimited BlackBerry plan is $29 by itself.