IF you have one of the memory slots open ( there are only 2 total) you should be able to use any 184 pin dimm.
If you can, try and use one that is PC2700 (333 Mhz). You can use any 184 pin modules that are slower, but you won't get the best performance out of them.
I have some ram cards from an old computer. How would I know if they can go in my Dimension 2400? Does it have certain numbers that I must see? Thank you.