1) Is this an iPhone 4 or 4S? Huge difference in this realm. The 4 is carrier specific. The 4S is the same hardware for all carriers but is locked by most of the carriers.
2) As others mentioned, you'll get charged more going from a crap -- sorry, I meant SIMPLE -- phone to a smartphone. It depends n your plan. I know the new share plans are $30 per regular phone, $40 per smartphone, meaning another $10 to you. On the old family plans, you keep the same minutes but have to purchase another data plan for $30/month.
If you do add your wife, I would consider the shared plan. I think 2 GB would run you $140, which includes unlimited talk and text plus use of the mobile hot spot feature.
If its an iPhone 4 you are talking about and it's incompatible but your wife still wants an iPhone, you can get a 4 for free with a two year contract. You can probably find a refurb 4S for $49 if you look hard.