Sidekick 2?

I just got a Sidekick 2 and I was wondering if you have to have the internet to have AIM. Do you have to have the unlimited internet to have AIM or can they just use your texts? So, let's say I have the 400 message bundle and I send 5 IMs. Will I only have 395 messages left, or will I be charged extra for the internet? Thanks!!