Getting a new phone for T-Mobile, 3G and 4G question?

I have a very basic phone for t mobile. Its just a slide up phone. I dont think I have 3G on it either. Anyways, next week the HTC Sensation is coming out and im gonna pick it up. So i was wondering about the SIM card. I dont anything about this, so, will i be connected to 3G? The phone is supposedly 4G, but i hear it really isnt. But lets just say 3G/4G. Im also gonna add the data plan for unlimited internet for $30 a month. So how will it work? Will i have to get a new SIM card to connect to 3G/4G? Or will I be able to connect to it just fine with the SIM card i have along with the internet plan? If i have to get a new SIM card, will i have to get a new number?