For recording tracks for OpenStreetmap, you will want to record the tracks to the card, and set the recording frequency as high as possible (1 point per second). Setting to WGS84 is not necessary, as that's what gets recorded into the GPX, regardless of what is actually displayed at the time.
For recording the location of roadside features (pubs, phone boxes etc), I find it easier to ride in a little loop than record a waypoint/mark. A small voice recorder is also handy if you have one.
You get one GPX file per day, which can be up to 4-5Mb. If you are still riding at midnight, the end of the ride will be in the next day's GPX file. If you turn off and turn on, you get separate track logs, but in the same file. The files have to be downloaded via USB mass storage.
Minimum is to just upload the GPX files onto OSNM for someone else to process. I usually process the files directly to mapped roads using the JOSM editor, without uploading the GPX.
Once in a while, I get a corrupt GPX file. This usually turns out to be because a bit of an earlier ride has been appended onto the end of the file, or even occasionally into the middle. Opening the GPX in internet explorer will tell you where the error is. A text editor that will handle many thousand lines and allow you to go to specific line numbers easily is handy for fixing the errors. I use WinVi.
