[groovico]

Your links/dropdown would have to put a variable into the user session and make it available to smarty to show the selected one on the switch.

You need to alter the currency tpls so they switch around what is shown depending on the session value.

X-cart only supports two though, you'd need to do more custom stuff to get more in there, and you also have to make it clear to the customer that secondary currencies are only approximate.

You can't change the base currency otherwise it messes up the orders unless you do more wizardry with real time conversions from another source.