I just installed a Linksys Wireless-G USB adapter in my Win x64 box, and it installed right from the git go. I just connected Linksys, booted Win x64, and directed the driver installer to the directory where I had extracted the Ralink drivers. The thing picked up my access point right away.
This is version 4 of the Linksys Wireless-G USB adapter. I have had version 1 for a long time before my Win x64 install, and it does not work under Win x64.
There is no way for one to tell from the box which version of Linksys Wireless -G USB adapter that is inside. It might be a crap shoot. I bought my Wireless-G from Buy.com, so I guess they don't have earlier versions.
Note that, in the .inf file that comes with the Ralink drivers, there is a list of other brands of wireless USB adapters that uses the Ralink chipset and probably works in Win x64.
I say "probably works" because I really don't know.
The string of letters and numbers, like "USB\VID_148F&PID_9020", are the numbers you will find under the Details tab of the Properties of a device in the Windows Device Manager. The VID for my Linksys that shows up in Device Manager is the exact same one as the very last one on this page.
I do not know if any brand of wireless USB adapter listed below, other than the Linksys models, can be found or purchased in the US or elsewhere.
As a general proposition, unproven by me, if the manufacturer or brand listed below has a wireless USB adapter (a 802.11g unit), it probably has a Ralink chipset and, accordingly, will work in Win x64 using Ralink's 64bit drivers.
Amigo_1 USB\VID_148F&PID_9020
Amigo_2 USB\VID_0EB0&PID_9020
Amigo_3 USB\VID_0EB0&PID_9021
MSI:
m2500usb_6861 m2500usb_6861 USB\VID_0DB0&PID_6861
m2500usb_6865 m2500usb_6865 USB\VID_0DB0&PID_6865
m2500usb_6869 m2500usb_6869 USB\VID_0DB0&PID_6869
SMC USB\VID_0707&PID_EE13
ASUS:
ASUS USB\VID_0B05&PID_1706
ASUSGEN USB\VID_0B05&PID_1707
ZINWELL USB\VID_5A57&PID_0260
VTech USB\VID_0F88&PID_3012
SURECOM USB\VID_0769&PID_11F3
Gigabyte USB\VID_1044&PID_8007
Siemens USB\VID_0681&PID_3C06
BUFFALO:
BUFFALO_5E USB\VID_0411&PID_005E
BUFFALO_66 USB\VID_0411&PID_0066
BUFFALO_67 USB\VID_0411&PID_0067
Sagem USB\VID_079B&PID_004B
Linksys
WUSB54GPv4 USB\VID_13b1&PID_0011
WUSB54Gv4 USB\VID_13b1&PID_000d