Shouldn't you be asking them this question? The poker sites should know whether they accept prepaid Amex cards as a deposit method.

Amex will not deposit directly to a Poker site, so you need to find what third-party processor you can use that takes Amex, and then what sites use that processor.

I'd start by deciding what site you prefer to play, and check out what you can use there (ePassporte, clicktoPay etc.) and then see if you can fund those with an AmEx.

Another option... Google is the best invention since the Internet.

http://www.google.ca/search?q=Poker+online+Amex