It was on by default in 10.5 and didn't have a GUI option to turn it off till around 10.5.2. So if you're running 10.5 and don't have it then you can't with your card. I have never tried to run 10.5.x on any system that didn't at least support QE.
Not sure why you would want it transparent anyway as most people hated it which is why they built in a check box in the desktop preference pane like shown below. I run 10.5.8 by the way..
EDIT: Found this info..
"Translucent Finder Menu on 450 MHz Power Mac G4
From Casey:
Good afternoon Dan,
I was reading the article regarding the translucent menu bar in Leopard being tied to the speed of the processor itself. I don't believe the processor totally controls whether the menu bar appears translucent or not. I am currently running 10.5.1 on a Power Mac G4 that is stock other than the upgraded GeForce 6200 video card, larger hard drive, and dual-layer burner. The processor is rated at 450 MHz, and I only have 384 MB of RAM installed. I have had the translucent menu bar from day one of 10.5 install.
translucent menubar in Leopard
With that being said, I believe that there is more to it than just what processor is currently installed.
Casey
Casey,
At this point, we have one G4/450 user (you) with a GeForce 6200 with the translucent menu bar and one G4/500 dual with the same video card and an opaque menu - but translucent when a 1.8 GHz upgrade is installed. I am at a loss to explain this.
Dan"
Source:
http://lowendmac.com/mail/0801mb/0102.html#1
To sum up.. some think the CPU needs to be a certain minimum speed for the transparency and some think it's a non-quartz extreme/core image card that stops it. There is evidence to show both sides could be right. Hard to say..
My Sawtooth with a 1.8GHz G4 7448 and a Geforce 6200 that supports every graphic option/feature in 10.5 does allow the transparent bar but as you see in that small screen snap I don't have it on. I personally hate it. To each their own.