My ecommerce store tends to deal with American customers so I thought it would be a good idea to deal in American Dollars instead of Pound Sterling. Is it possible to do this?

The Currency is selected per Order Type.

If you use a different Website per region you can use different OrderTypes with different Currencies, Email Objects and Email Addresses.

You can get the same effect by using Language Versioned Webpages if you are only using 1 Website. You can have different Atom Configs per Language so that is where you setup the different Order Types.

Hope this helps you.