I recently upgraded to a 310XT from my 305 which had some problems after running for 2500 miles...
Now I wanted to import my history into Garmin Connect. I tried to export the whole History in both formats supported for Export in the most recent version of Training Center, and then to import it using the manual upload feature.
The .tcx and .gpx files differ in size (one is 18, and the other is just 5 Megs) but that's probably too much to import in one shot anyway. So what's the file size limit? Or is it a timeout causing the problem (so I guess working on a veeery fast connection could help). I tried to upload using a 512 Kbps connection, so it would take a while, anyway.