The OG has one mb of RAM. 576k is reserved for the space that applications need while they are running. Up to nine apps can run in this space at the same time. When the space gets full, GEOS automatically shuts down the "oldest" apps to make room for the new ones. There's no way to tell how much of this space is free or in use, but it doesn't matter because GEOS manages this space automatically and you can't use it for anything else anyway. Then there's 32k video memory. That's used for the screen display and again you can't use it for anything else. Then there's 416k of user ram, about 8k is used for overhead for the ramdisk and the remaining 408k is space for your documents and other files. You can see how much of this space is available by looking in the Transfer app or in Setup.
<<< 2. When I first bought my Omnigo (before I had entered any data such as addresses) the Transfer app indicated 183 kb Used and 225 kb Free; any idea why almost half of the available memory was "used"? Finally, >>>
You probably ran every application on the device. Each one created a state file (from a few k to about 20 k each). Doing a warm boot (SHIFT+ON+NEXT) will delete the state files and free the space they are taking up. Deleting state files won't hurt anything.
<<< 3. Sometimes when I delete some data (e.g. Phonebook entries) my Free memory seems to DECREASE rather than increase; any clues why? >>>
Probably you're running additional applications and creating new state files between when you delete data and when you look at the remaining space--like you have to run Setup or Transfer to see the space, for example. Doing a warm boot will delete the state files and free the space they are taking, without doing any harm to your document files.
Steve