AbleCommerce Shopping Cart Software

I'm trying to find out if there are any additional fees associated with having more than one currency type on the site..

Our primary currency is British Pounds Sterling - we provide Euro and Dollar options.

On the client side, all payments APPEAR to be in the client's selected currency.

Under the bonnet, however, payments are in the primary currency type - i.e. GBP

Q1. What happens, however, when a European Credit Card Holder makes a payment for what appears to be a euro amount on the site - their card gets charged for the GBP amount, so will they get charged currency conversion fees by the credit card company??

Q2. Can you push payments through in the user's selected currency??

Q1. What happens, however, when a European Credit Card Holder makes a payment for what appears to be a euro amount on the site - their card gets charged for the GBP amount, so will they get charged currency conversion fees by the credit card company??

Please query credit card companies about what their policies are in such cases.

Q2. Can you push payments through in the user's selected currency??

There is no out of box solution for this case. One way to do this could be to setup one gateway one currency and another for the other currency. Then based on the user's selected currency we can decide on run-time which gateway to use to process the transaction. Some persons already have tried some similar jobs. Please read following thread
viewtopic.php?f=42&t=10123

Mazhar,
Can one payment gateway not handle the various currencies?

I'm using a bespoke RealEx payment gateway, and they can certainly accept Euro, USD, and GBP payments.

Is there an AC related issue why the euro amount (on a site that has GBP as the primary currency) cannot be passed to the payment gateway?

You don't need to have different payment gateway dll for USD,EUR etc.
Charging the user in local currency(say EUR and store currency is GBP) so that he will not be charged conversion fee means store having one more EUR account with the gateway provider.

Now make a duplicate entry of the store record in the payment gateway table and have EUR account username and password in it.