Anyone remember slot-A Athlon?

I bought one of the first slot-A Athlons with a Gibabyte GA-7IX motherboard.
It has served me well but entually the onslaught of virus checking and spam
filtering has slowed it too far. I thought a cheap upgrade would be to
replace the 500mHz cpu with a faster one.

I got a 900mHz slot-a Thuderbird, fitted it, but it appeared dead - case fan
idle, no video signal, minimal disk activity, CPU fan running. I'd seen
reports on the internet that the motherboard was compatable with the
Thunderbird Athlons, but the list of compatable motherboards at the AMD site
does not list the GA-7IX. so I thought perhaps I had just purchased the
wrong CPU.

I then got a 750MHz classic Athlon but when fitted to the PC it again
appeared dead - no case fan rotation and no video signal to the monitor, but
CPU fan running. This CPU should have worked, same series and a frequency
within the motherboards range, and I find it strange that it shows the same
combination of running/non running symptoms.

In both cases the PC came back to life when the original CPU was
reinstalled, so I think it is unlikely that I have killed both CPUs through
mishandling.

What I need now is some advice from someone who knows more about the subject
than I do, do you have experience of the Gigabyte GA-7IX ?

Should it be possible just to change the CPU and then swich on for a
woking PC, or do I have to change some jumper or BIOS settings?
Are the symptoms I described typical of a dead CPU; I find it strange
that one fan should run and the other not.
Is there anything obvious that I'm missing?

Bios version is F3. Board version is V1.0. Any help appreciated. TIA.