Well, you were on Linksys' site. They are the manufacturer of your device. If they don't have a 64-bit driver, I doubt if one exists yet. Maybe you'll have to look for hardware and drivers from D-Link, Netgear, etc.
Edit: I just discovered a site called PlanetAMD64 which is focused on 64-bit OS running on AMD processors. They have forum, with a section on
http://www.planetamd64.com/index.php?showforum=17. Might be worth checking out. Here's a post specifically on your issue:
http://www.planetamd64.com/index.php?showtopic=19561.