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What I would do is have the default currency of your most buyers and just promote the fact that you can do both and don't forget to promote where you ship to with costs.
I am not too sure about the tech side of changing your currency is you are using something like OScommerce then its as easy as switching something on in the admin panel.
Also, bear in mind what currency your payment processor uses as this can affect the final amount the customer is charged if it has to be converted. You will need to ensure the exchange rates are kept up to date.