Both machines are Del 2400s.
The original machine's RAM is labelled:
Hynix 256MB DDR, 333, CL2.5
PC2700U-25330
The RAM I'm trying to add from the decommissioned machine is labelled:
Infineon 256MB DDR, 333, CL2.5
PC2700U-25330
There is one vacant RAM slot right next to the one currently in use. I've checked that the RAM is slotting in correctly.
I also swapped the RAMs around so using the Infineon in place of the Hynix
and the computer booted correctly/ Howewver, then adding the Hynix to the spare slot, it stopped booting up after about 10secs.
Many thanks