Dell's don't need "specific" memory, which is clear from my previous post. However the motherboard operating your Dell may require a specific type of memory, it may also require it in different modules e.g. If you want to increase the memory to 512mb you would require 4 x 128mb and not 2 x 256. Also some boards will not allow you to mix unequal amounts. When I added mine I left in the original 64kb (but not in the same bank) and added the 256mb block, some systems will not allow this and you would need to remove the 64mb. All this is explained very well on the Crucial website, worth reading in any case if you are interested in these sort of things.
Regards
FBW