I'm actually thinking to add currencies besides only USD and EUR. At the end of the day we don't have any particular love for either, since our costs are all in JPY. The biggest issue is to not clutter up the interface...
What I've seen in some e-commerce shops is that they automatically change the currency based on the customer's location and have a small option on the bottom or top right of the site displaying available currencies. For your site a dropdown menu/box for the currency to the right of the price might be best.