I also noticed the “Show 3D preview on second monitor” in Preferences|Graphics Options doesn’t stay checked (latest beta x64.) [edit] and the preview doesn’t appear on my second monitor.
The conserve memory seems to work for me and the box stays checked.
There was an issue where the checkbox for mem conservation was not participating in the “changed” notification so WM didn’t think that you had actually changed the value. If you modify the Conserve memory checkbox, then slide the resolution slider back and forth, it should stick.
Note that this is fixed in 2.3 beta3.
The Show preview checkbox is obsolete with the introduction of the generic secondary windows in 2.3. I will remove that option from the dialog.