Depends on what you're converting.
I've been tinkering with GPS-Babel 1.3.4 with the Track logs, and Geocache data. (feeding the info to a 2GB SD card in the 500LE)
I've been able to use it to review the track logs under DeLorme Street Atlas USA, after conversion with GPS-Babel, and replay the track log on the map with little problem (occationally need to hit the FFwd to read the next position.)
direct route to the download page:
main website: http://www.gpsbabel.org