I don't have this network card so i can't test.
But i guess you can get rt73usb-firmware from ralink homepage: http://www.ralink.com.tw/support.php?s=2 (You may accept the Licence agreement).
Unpack the file with unzip :
# tazpkg get-install unzip
Copy the rt73.bin file in /lib/firmware directory.
You will need linux-wireless wireless_tools packages too.