How do I get a GoPhone SIM card into my Samsung Captivate?

I'm tired of spending ~$80/month on a plan that I don't really use. I currently own a Samsung Captivate with all the bells and whistles of a contract. I would like to get a GoPhone SIM card, with the 10�/minute plan, and stick it in my Captivate. I'd like to keep my current number as well, but I'm getting confused as to what order I should do everything in, so I don't lock myself out. Any hints from someone who's already attempted this?