Hi David,
No, Garmin Mobile XT can only be used on the pre-programmed memory card and it will not work if the contents are copied to a non-original memory card. FYI: The pre-programmed memory card has a license file that will only allow the application to be launched from the original memory card (unique card ID)
I am not aware of Garmin topo maps for PDA / Smartphone. Their topi maps are always available and designed to be used with their own Garmin Mapping Handhelds: https://buy.garmin.com/shop/shop.do?cID=145
For topo maps, pls consider Fugawi: http://www.fugawi.com/web/products/fugawi_map_of_america.htm (also compatible with your Palm OS in the Centro).
Wrong Assumption: Centro does NOT have a GPS built-in and it is not a GPS enabled smartphone. So, you must also purchase a Bluetooth GPS receiver to be used along with Garmin Mobile XT or topo maps of your choice.
For Geocaching, I recommend: Cachemate http://www.smittyware.com/palm/cachemate/
Other possible alternatives (please check if they have a Palm OS version, I know they have a Microsoft Pocket PC Version for sure:)
Backcountry Naviagator, Beeline, and GPS Tuner