Ahh, Dimension 2400 -
Kingston and
Crucial both agree on what
should work in your PC.
I'd be surprised if you have to go into the BIOS to set the RAM quantity - it should simply register it in the POST and possibly leave a message on screen until you hit F1 or something (I'm not familiar with Dell).
Either one of the modules is duff or one of the motherboards is duff (at least the second memory slot).
SD