Since Magento has built in functionality for currencies, it shouldn’t be too hard to create custom currency selector and put it to the header. You might say this tutorial is for beginners, since it’s pretty much straightforward.

You might have noticed the ”Currency Setup” tab in Magento’s Administration under “System->Configuration” menu. There you should select default site currency, and besides that, all currencies you want to support.

Here’s a screenshot of that tab:

After that, you should go to “System->Manage Currency Rates” and set rates for currencies you’ve chosen before. You can use Webservicex to import currency rates from Webservicex service. Here’s how it looks like:

And now, after you’re done with initial setup, let’s go further with modifications to make that output shows in the header.

If you would like to move the currency switcher from the left column to the header, you can use the following instructions to do so!

Open /app/design/frontend/default/default/layout/page.xml (or under your theme)
Locate:

<block type="page/html_header" name="header" as="header">

Change to:

<block type="page/html_header" name="header" as="header"><block type="directory/currency" name="currency" template="directory/currency.phtml"/...</block>

Open /app/design/frontend/default/default/template/page/html/header.phtml (or under your theme)
After:

<?php echo $this->getChildHtml('store_language') ?>

Insert:

<?php echo $this->getChildHtml('currency') ?>

Save, Upload + Clear Cache
Make sure you have imported or manually entered your Currency Rates otherwise the switcher will not appear!

Its as simple as that!!

from:

http://inchoo.net/ecommerce/magento/how-to-add-currency-selector-to-magentos-header/

http://www.pauldonnelly.net/magento-move-currency-switcher-to-the-header/