This was fixed in Froyo. Anything over 3*160 (480 characters) will convert to an MMS prior to Froyo.
You do have a couple options.
- Use a 3rd Party Messaging App, like Handcent, for example.
- See if you are able to install this "Hack"
- Root your phone and flash a Froyo Rom
- Wait for a Froyo update from Samsung
Edit: Some additional information
Anything over 160 characters, technically, is no longer an SMS. The stock Messaging app, and others too, allow for "longer" messages but it is just splitting up the messages in to 160 characters. In Eclair (Android 2.1) a message will be converted to an MMS after it exceeds 480 characters. At that point, the default Messaging app will no longer split up the messages, instead it just sends it as one MMS. If you are using a 3rd party messaging application and find that you are still being charged the MMS rate, it may be that your carrier is grouping messages that come through "at the same time".