I suggest that you use the KINGSTON RAM configuration tool - I recently upgraded a Dell Dimension 4100 for my brother-in-law and went from 128 to 512 sweet as a nut! Sourced Kingston RAM via ebuyer.co.uk
Also went from Windows ME to XP and trebled the HD capacity, but that's another story.
You should be able to find the Dell part number on the module you've got, then check the same part against what Kingston says the OEM part number is, then buy the next size up. Doesn't always work, though, as OEM part numbers and upgrade / replacement part numbers are often different.
SD