Create a new list with the currency codes and conversion values, and use a lookup column to pull the right values. This will scale far better, anyway, since conversion rates are far from constant.
I'd also suggest that you implement the lookup column as a Site Column so you can use it later in additional locations in your Site Collection.
Just because we use flat lists in SharePoint, it doesn't mean that we can't attempt to follow good relational database approaches.
ADDENDUM: I'm so used to using Data View Web Parts (DVWPs) that I didn't think enough about this answer. It turns out that the value in the second list won't be available for use in a calculated column. You can use a DVWP to do the calculations, though. You can either pull the exchange rate value across with the lookup column in 2010 or use an AggregateDataSource in 2007 to look it up in your XSL in the DVWP.