G5 desktop MAC

I am running Cubase LE4 with an ESU 1808 interface, MOTU microLite, ESQ-1, ASR-10 and a microKORG all going out to a pair of Mackie MR8’s.
The G5 can only run OS X 10.4.(one more number which I can’t remember)
I recently purchased a Mackie Control Universal (the older one). All the instruments MIDI through the microLite. The controller MIDIs to the audio interface. I can build a MIDI track in LE4 which lets me control any of my keyboards via another one. i.e. I can use the microKorg’s apeggiator to control the ESQ1
I have been going through the Cubase Tutorial Projects to get a handle on how the Mackie works with everything as I can’t seem to find a lot of info on it. I am still trying to work out what I am seeing when I press either the Instrument button in the Assignment section or the Action section of buttons.

Anyway, all goes well until I get to about the Mixing 8 Tutorial project. It doesn’t all come to a crashing halt, but it would appear that it is all too much for the G5 to handle. Missing notes, controller doesn’t update the time clock display correctly, spinning beach ball etc. iStatPro shows the CPU graph as going to 100%.
I am guessing that I am limited to doing things with only about 4~5 tracks in them as if I add automation to any of these, I will just simply overload the CPU again?

If I was to look at updating Cubase, do I also need to look and getting a pretty high end MAC?
I will admit that I thought that the G5 would be able to handle more than what it is.
If I use Cubase for MIDI tracks to control the keyboards, am I still going to run into trouble with the MAC that I have?
Have I just shelled out for an 8 track controller with a system that can’t handle 8 tracks?
I am asking these questions as I am not proficient enough yet to simply set up some sort of project to test all of this?
The next thing after that would be finding the money to upgrade.