Do you have a USB cable to connect the GPS to your PC? There might be a way to do it with a card reader, but have never used one myself - only a cable. Here's how I do it -
Load the MapSource software - should be on the Topo US CD. Attach your GPS to your PC, and do a Send or Receive - if the MapSource software finds it, the model name/number will be displayed. Now - here's the slightly tricky part - the maps are divided up into quadrants, and they're displayed when you zoom out to a certain resolution. Once you think you're there, fiddle with the icons along the top row - one of them is a Map Tool. Once you find the Map Tool, move to the upper left are of the section of the map you wish to download, and holding down on the Control key, highlight those quadrants you want to transfer - the high-lighted ones will turn pink. Finally, hit the Send button, and be patient - depending upon the size of the total selection, it might take awhile. Also - if that process seems to work, Save the map selection - it will save you time later on if you want to add or delete quadrants.
Am quite sure transferring the maps will overwrite what already exists in the GPS - at least that's the way it works with my 60CSx.