I recently upgraded my memory from 256MB x 2 to 2 GB x 1. I just went into the local computer shop and asked for a RAM and they gave me what I asked for. I just told them I had a Dell and it worked. I tried putting one of my old 256MB's in and i got an error message, so i think 2gb is the max.
So just go into your a local shop, ask for memory, tell them your computer and you will get one. Just make sure the memory you get isnt too big for your computer.
That probably didn't help, but yeh