If you get an unlocked GSM phone, you can indeed use it with any GSM provider, regardless of model and brand. These days they're all quad band world phones so you no longer need to worry about supported bands for voice calls and EDGE (slow) data.
Unless you contact your carrier and change your plan, you do not need to get a data plan. Carriers only insist on a data plan when you buy one of their phones. However, you will probably want to turn off the cellular data option on your new phone to make sure it doesn't use your carrier's data network instead of WiFi.
-------------------- Lisa Gade Editor in Chief, MobileTechReview