Been trying to install RH9 on P4 - FX5600 Ultra system, RH9 installed ok in text mode but would not boot to x-windows monitor went into 'no video signal' mode.
So! - installed RH9 on lo spec system I knocked up from old bits - Duron 800 - Geforce2 GTS. NO problems at all. Installed latest Nvidia drivers with a lot of help from the kind contributors here, got the Nvidia logo on boot so assume drivers installed ok.
Reset display setting to match FX5600 ultra spec - shut down - install hard drive in FX5600 Ultra system. RH boots to its loading screen but the computer resets when the kernel loads!
Anyone got any ideas?

You probably got a problem somewhere else in between the two boxes.I got a system with a onboard (mx something) nvidia chip and a FX 5600 and can switch between the two without any problem.
Your original install didn't work because it uses the xfree driver and that doesn't support anything FX.
An error message from when it dies at boot or where it dies would be helpful.Don't know how RH handles the kernel - like if it uses different kernels for AMD and Intel.Probably also tries to load the wrong drivers for the motherboard.

No error messages - screen goes blank as soon as the countdown timer reaches zero and (presumably) executes the first line.

You could try to just flip in the first RH cd.They got some rescue system in there.I know thats very fague but I didn't use RH since 6.0.I am pretty sure that your problem right now is that the kernel doesn't like to work with the motherboard.Or try to post at the RH forum or search for redhat and rescue.