iPhone 4 Data Use Question

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Looking at getting an iPhone 4 with a data plan and costing it out.

Just wondering how much data would be used day-to-day on e-mail, browsing, monitoring an Apex monitor, and other use of the iPhone 4, if most of my browsing was done on my computer or iPad through the home router and only using the iPhone data browsing during travel and when not at home or work.

Also does the thing automatically switch from the phone company's wireless data access to your wireless router if you are within range of your router?