Thanks Eric,

I did upgrade my iMac memory using Other World Computing over a year ago.  For my particular "early 2008" iMac, it was determined by Apple, given the SN, that the maximum memory was 4 GB, not 6 GB.  That is probably the reason why they also ruled out attempting to put El Capitan on my particular iMac.

I've poked around with my iCloud Drive.  I find it is only storing and syncing Pages, Numbers, Keynote docs.  There is nothing there from my Notes or other standard Apple apps that come in the Apple system.  My preferences show that all these apps are checked to be actively stored in iCloud Drive and synced, but they are not. I also noticed that iCloud Drive doesn't seem to recognize hierarchical filing within Pages, Numbers. It counts as a single "item" any file folder, despite there being multiple items within the file, and sub files.

I did not try turning off "syncing" under the iMac's system/preferences. I tried turning off just one of the Apple apps from iCloud syncing, and it warned me all docs from my iMac for that app would be deleted.

I plan to next ask the techies at my local Apple User Group for advice. I suspect the answer will be get a newer computer.  That doesn't answer, however, why iCloud Drive doesn't store the Notes, Contacts, Email in the cloud or sync the entries between iMac and the iOS devices.

Fortunately for the iMac backup I have a LaCie drive doing so through Timemachine.  But the docs or the entries I do through my iPad or iPhone are not getting synced into the iMac.

Thanks to both Eric and Lanny!

You convinced me I should get the 6GB upgrade kit for my iMac, April 2008.

I've ordered it already.  I do want to upgrade then to El Capitan.  Save myself from buying new computer...!

I did more digging and comparisons of the syncing on iCloud. iOS (iPad and iPhone) syncing is 100% through iCloud. The problem is with iMac not syncing on Notes anymore. iMac does sync new Calendar and Contacts updates from the iOS through iCloud sync.  I'm hoping El Capitan will clean up whatever the iMac is doing to fail on Notes.

Eric, I also did as you suggested: I clicked off the apps under preferences/iCloud/iCloud Drive that I have been concerned about.  I waited some minutes, and reclicked as active under iCloud syncing.

I do notice that iCloud Drive does not accurately show all of what is in iCloud files for Pages, Numbers, Notes. 

For now I will focus on seeing if the 6 GB Other World Computing upgrade followed by El Capitan upgrade from Yosemite get the April 2008 iMac to work with iCloud on Notes.  Also, does iCloud Drive show all recent Pages, Numbers, as it is supposed to do.

It is possible that the purchase of the new iPad and iPhone at my local AT&T store two weeks ago kept the iOS devices syncing together, but lost iMac syncing on Notes -- yet it does sync on Contacts, Reminders, etc.

I've learned so much thanks for your very helpful suggestions and pointers, from Lanny and Eric.  Kudos!