click here for information relating to the Dell Dimension 2400.
Step 2 about half way down the page explains it ie. it will work with only one memory module fitted.
I've got the same machine which originally came with one module of 512mb, (as opposed to two of 256mb). I've added another 512mb without any problems, (touch wood, famous last words etc).
The maximum for the machine, stated by Crucial, is 2048mb.
(The above assumes your machine is a Dell Dimension. Otherwise, look further on the Crucial site and put the specs of your particialr machine in.)