Hi Mark,
I didn't check your link on your original post. Sorry.
There is one other model that is a 400 MHz model that will accept cards with an ADC connection port.
That's the Gigabit Ethernet model.
You do not have this model Mac.
The table in the link we both have show you what video connections the and locations for those on the back panel.
Clearly, your PowerMac doesn't have an ADC connector for the card that is already installed.
So, you can conclude that you have the AGP sawtooth model that only uses VGA or DVI connection.
You can give me your PowerMac's model number to be sure.
It's located on the back panel somewhere near all of the connections.
That'll clinch, for sure, which model you do have.
You will have to purchase an ADC to DVI adapter when you decide which card to update to per JapanMac's suggestion.
The adapter doesn't affect video performance.
Just allows for a connection change at the computer.
This is about as straightforward as this gets.
The absence of an ADC connection port means you have a Mac with a VGA and/or DVI connection instead.
An adapter will be needed to use the 23 inch monitor you have with the card you have installed now or any future video card upgrade.