What shopping cart software do you use on your site?
If you have none, take a look at the www.oscommerce.com
or something similar.
All you need to do now is set up your (BTC) account and go from there. You can probably set BTC as one of the currencies and update the price regularly.
If you are USD based then get your price updates from http://mtgox.com/code/data/ticker.php
or any other BTC exchange sites.
BTW, MtGox also has a Merchant Services https://mtgox.com/merch/about
- it si possible, that this is all you really need.
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