Backup old MAC and replace MAC OS with Windows or Linux

RAM has no bearing on your backups.

Just make a tar backup of all your data, then if you're going to Linux use any compressor (gzip, xz, zstd). Going to Windows use zip.

Replacing the OS with Windows (ouch) can be done + will likely work best on fairly new minis. Older minis will likely have more + more problems as time goes by.

Replacing OS with Linux is also easy. Only complication is best to start with an Ethernet connection for your networking, as you'll likely have to manually install a WiFi device driver, as Linux seems to have problems (at least on older minis) trying to correctly probe WiFi hardware, then correctly determine which WiFi device driver to install.

Aside: If you have no Ethernet connectivity, you can transfer the WiFi device driver on a USB stick.

I do not have experience with tar backup.  Is there not an easier way to backup data from an old Mac?

hello,

first of all you have to backup your MAC files and folders to an external hard disk or to the cloud if you have enough internet data.
For online backup you use google drive, one drive, dropbox and Icloud but there is a limitation of how much you're allowed to store without paying additional fees.
For the offline backup:

• connect your hard drive
• Format it in case needed
• open Time machine and turn it on
• select the drive that you want to back it up
• you can use to encrypt in the disk (better in case the data is confidential)

For the restore

• connect the disk that contains the backup
• turn on your mac and press (⌘)-R to start up from Mac OS recovery
• select the backup and restore it 

Time machine is by far the easiest method for most Mac Users to back up data, but will only be readable from another Mac.

How old is your Mac?  Which OS does it currently have?  Why are you switching?  Is it too slow?  Maybe you just need replace the Hard Disk with an SSD.  Maybe you need to increase the RAM.  Switching to SSD will make it seem about twice as fast for some things you do.  Adding RAM will increase the speed because it won't be going to disk as often.

If it's an Intel Based Mac, then you can just use Bootcamp assistant to create Windows.  After you install Windows, you can erase the Mac partition and expand into it, if you so desire.  Don't use Linux if you're not really familiar with it at all.  There's going to be fewer people to assist you when you need help.

If you have a Power PC based Mac, you will only be able to replace it with Linux if you need a "newer" system.  I suggest you just keep is as a Mac or get rid of it.

and note theat you should not expect to restore the apps. merely the files. and that is probably enough.

if you do not want to bother with the command line, you cxan install 7zip ( 7zx ) which handles most archiefe types, of just backup to an usb key. for large volumes you can also copy directly from machine to machine over ftp for example

7Zip for mac comes as command line, not GUI, just like linux.  You should either get The Unarchiver or Keka if you need GUI.

You can restore some Apps, mainly those that come as complete .app drag and drop copy packages, rather than those that need installs.

Mac is BSD based Posix Unix already.  If you just want linux functionality, then install Homebrew and add your favorite Linux utilities for free.

7zx ix the gui version for mac.

and indeed many bsd and even linux apps can run rather easily in a chroot or directly using an adequate LD_LIBRARY_PATH

7zx was a GUI version for Mac.

7zx is only available from Download aggregators, not the official 7zip page.  The 7zip page points to Keka for official Mac 7zip support.  It depends on how much you trust the 7zx, which was only available briefly on the 7zip page long ago.

They had not supported a GUI for a while after 7zx was removed, until Keka.  That tells me the 7zx isn't updated and was someone's personal project that has become unsupported and is based on old code.  That isn't necessarily bad; it's just additional information you should have to make your decision on whether you should use 7zx.

I recommend using Keka over 7zx, because that's the supported version linked on the 7zip Downloads page.  This is why I use the command line version on Mac; they don't roll their own version of the GUI for the Mac.  I've been using 7zip since its very early days when I replaced an idiot "sysadmin's" cracked warez copy of WinRAR on a few hundred systems.  It was Windows only back then, and unix/linux had gzip, and Mac had The Unarchiver and gzip if you installed Fink.  It was only later that command line 7zip came out for linux then OS X.  The 7zip Mac GUI support was always 3rd party and came much later.