Can I use my iMac (PowerPC G5 2.1 GHz, Build 9L31a, Serial # W855104UTAR, VERSION 10.5.8) as a monitor with new Mac mini.
If yes, how to do it?
You either must use a monitor to initially set up the Mac Mini, or use Target Mode to boot into its hard disk, and create these settings, but:
You turn on Screen Sharing in the Sharing panel in System Preferences in the Mac Mini. In my case I also give my Mac Mini a fixed IP with DHCP with Manual Address in the Network panel.
Then you boot your iMac and in the Finder in the GO menu to Connect to Server... and enter: "vnc://192.168.0.148" but replace 192.168.0.148 with your Manual Address for your Mac Mini. You log in with your name and password for the Mac Mini and your Mac Mini will now appear in its own window.
Go to Full Screen and now you can running your Mac Mini with your iMac's keyboard, mouse and monitor.
I have been using my mid-2011 Mac Mini in this fashion since I purchased it.
As a backup, I have an HDMI cable connected from my Mac Mini to my HDTV to allow for "monitor viewing" when and if needed.
I have an older Mac Mini running Snow Leopard as part of my home entertainment center, and with its own IP, I also control it in this fashion without a monitor (although I can "view" it on my HDTV through its component video input, when needed).
This is known as a "headless" mac mini.
In my case, instead of an iMac, I use my MacBook Pro.
I think the OP just wants to use it as a monitor by direct connection. Using screen sharing (on a PPC model - is that even possible?) is not quite the same thing. A monitor running at wireless speed is very slow.
I always try to interpret an OP with the standard of knowledge somewhere below that of Kappy! Hence, I read the OP broadly to consider options that may not have been suggested with a strict interpretation of the question asked... But is well within the universe of less knowledgable Mac users, who, in all likelihood, would be the ones asking the questions (and not answering them!).
I'll let the OP decide if my answer helped him or not...
That being said: I use both of my mentioned Mac Minis headlessly over a wireless network without the slow down that you fear (I am NOT running fast scan video over this setup!).
A PPC Mac may require the installation of a mac compatible VNC client, if Screen Sharing is not present in Leopard 10.5.8.
Oh, that's not the same as Screen Sharing, i.e., what is now built-into OS X, other than it's a form of VNC. Since the required software must be on both machines (Chicken client & server) it should work. But otherwise there's nothing compatible on the old machine that would sync up to the server/client on the new Macs. That's what I was talking about.
Now that I think about it, my (now-ex) wife had a mini-LAN run on an iMac G4 with Tiger Server installed on it.
I used to be able to remote screen share to it with a simple Finder to GO to Go to Server... and her virtual (external) IP entered into the command: vnc://[IP]
On her end, I made sure her router was modified to allow for port 5900 (VNC) to go to the internal IP of her iMac G4 and it worked flawlessly.
Using afp://[IP] I was then able to mount her server and move files to it, as well.
So, YES: you can screen share to a PPC running Tiger!
Of course, you are asking the opposite question; can a PPC Mac running Tiger screen share to a modern day Mac and I must assume that opposite would still be true (I may have actually done it, I just can't remember...)