UTM Home RAM Limitation with 9.712-13 ?

UTM doesn't have hardware limits, XG does. UTM has an IP address limitation of 50 IPs.  XG is limited to 6GB memory and one quad-core CPU.

That´s good to hear, thanks for your quick answer!

So what do I have to do to make my UTM Installation "detect" more memory so I can use it?

Well normally in the past, when you change hardware in a Linux environment, especially related to NICs, you would have to reinstall the system.  It's gotten a lot better with handling that aspect of hardware with the exception of the NIC.  Any time you change a NIC in that environment, there's no other clean solution than to reinstall.

I don't run my UTM in a VM environment and keep it as its own device, using VM for my backend servers.  Normally, you would see that as an issue with Windows and a 32-bit system.  The only thing that comes to mind is your install isn't utilizing a 64-bit installation, but that would normally relate to a Windows 32-bit install.  I don't recall Linux being affected by that.  You may have to enable DEP on your VM. If your hardware doesn't support that, you would have to have something like a /PAE switch command for the boot to see the memory.  I don't know what the switch command would be for Linux, but for Windows 32-bit systems, it was /PAE after the /fastboot switch in the boot.ini.

If you are 'hot adding' memory into a Linux 64-bit VM, or any Windows 32-bit VM, you can't do that. 

Well normally in the past, when you change hardware in a Linux environment, especially related to NICs, you would have to reinstall the system.  It's gotten a lot better with handling that aspect of hardware with the exception of the NIC.  Any time you change a NIC in that environment, there's no other clean solution than to reinstall.

I don't run my UTM in a VM environment and keep it as its own device, using VM for my backend servers.  Normally, you would see that as an issue with Windows and a 32-bit system.  The only thing that comes to mind is your install isn't utilizing a 64-bit installation, but that would normally relate to a Windows 32-bit install.  I don't recall Linux being affected by that.  You may have to enable DEP on your VM. If your hardware doesn't support that, you would have to have something like a /PAE switch command for the boot to see the memory.  I don't know what the switch command would be for Linux, but for Windows 32-bit systems, it was /PAE after the /fastboot switch in the boot.ini.

If you are 'hot adding' memory into a Linux 64-bit VM, or any Windows 32-bit VM, you can't do that. 

You were right, too. But there has to be something special with Sophos. Normally a 32 bit Kernel is not limited to 3 GB.

No, that is correct.  A 32-bit architecture OS only has enough address space for 4GB and it requires part of that total for communication with the devices and hardware in the system.  So in return, you are only allocated 3GB for memory space.  The other ~700MB or so, whatever the OS takes isn't available.  Windows acted the same way for 32-bit systems.  You didn't see it all the time in Linux, but it was definitely there.