According to:
http://www.3dvelocity.com/articles/win64compatibility/win64hardlist.htm
There are two vesions of the WMP54GS: version 1.0, which
is compatible w/ x64, and version 1.1, which is
not compatible.
It seems that card manufactures will sometimes change the chipset on
the card and give no indication that this has been done other than
changing the version number of the card.
I just bought one of these cards, and found that it didn't work on x64.
There was no indication on the box, or even on the card, of what
version it was. I guess that I have version 1.1 of the card based on
the version number of the install CD that came w/ it.
Regards,
Eric