I am developing an ecommerce website for a UK client whose market is also based in the US.
Can anyone advise me of the best way to display dollars versions of the prices (as well as pounds) and keep this info reasonable up to date?

Thanks in advance

to be honest, I think your question is more of a back-end technology question for a web designer. You may want to, if you have not already, post this question in the Database or Web-Design subforums.

I recommend zen-cart for this purpose. It support all main currencies and customers could choose which currency while they are shopping on your site.

I did not know that zen-cart offered these capabilities so thanks for the added info.