Hi amardilo, I would advise caution here, if you are using Thunderbird now. You will appreciate perhaps that Thunderbird uses industry standard MBX format for its mailboxes, allowing you to transfer your mail seamlessly amongst a large variety of email clients. Unfortunately, Apple's Mail does not use MBX format. It uses a proprietary format as of Tiger to enable Spotlight to more easily search email contents. As a result, once you go to Mail.app, you will have a much harder time moving your email out to another email client if you should decide to. It is not impossible, but it is not as simple as it is from Thunderbird. You might wish to Google this topic a bit and see all the solutions that are possible before deciding to commit your email to Mail.app.
This is NOT a Mail.app bashing thread. Mail.app is a great email client, but it does not use the major mailbox standard, so you need to be aware of this if you are headed that way.