At Admin Panel -> Settings -> Localization you can type the appropriate default currency sign and currency code for your locale. The currency sign is used in the normal display of prices, while the currency code is needed for payment processing. Here's a list of currency codes: http://www.xe.com/iso4217.php

Sponsorships will always be processed with your primary currency, but if you have topics that have prices associated with them then you can allow the topic submitter to choose between multiple currencies. Enter your list of alternate currencies on the Localization page under "Currency Options"... you'll need to enter the three letter currency codes, one per line.