Just a word of caustion..
What OS are you running on your PC? I tried for two weeks to get the USB download cable to work - Garmin even sent me a new one and it wouldn't work. Then I started really reading all of their documentation. In the "USB Data Card Programmer" manual it says it will work with XP. But in the "Mapsource User's Manual Addendum for USB Data Card Programmer" it says it will not work with XP. Well - to date - we (Garmin and I) have not been able to make it work on either my work PC (IBM Thinkpad with XP Pro) or my home PC (Dell with XP Home).