Display problem Powermac G5

Hoping someone can help me, I'm new to Macs.

I just bought a used Powermac G5; ATI Radeon 9600Pro, 2 hard disks. OSX 10.5.8 installed. Connected to a Dell 27" monitor. Looked nice, but as the max display resolution is 1600x1200, the dislay was stretched by the monitor. Looked about an found a 3rd party screen manager, ScreenXrefesh or something like that. Anyay, it allowed the creation of custom screen sizes.

I created a 1600x1440 mode, and rebooted. Oops. The Mac boots, but the display is just a soft blue screen with a cursor, nothing else. This happens whether I boot as usual, or supposedly in safe mode (its a PC USB keyboard, so I'm assuming the Shift key press is registered on booting. I have no idea how to map Command-S from the Mac keyboard to a standard USB keyboard to safe boot in single-user mode).

So I unattached the first drive, attached the second, and installed OSX 10.4. All good. What I'm hoping to be able to do is re-attach the 1st drive (which is in the B drive bay), boot from the 10.4 drive, and manually reset the screen resolution on the 10.5 installation by editing a config file.

But I don't know where the system holds screen resolutions, and whether the file is editable. I'm /presuming/ it is possible?

Which file do I need to edit (or zap), and where is it located in a 10.5.8 installation?

Many thanks