The following websites offer RAM compatible with Dell systems and generally list compatible parts by model:
Both Crucial and Offtek have a system scanner for determining your system and telling you what's currently installed and what parts they have available for you to upgrade.
Note older memory modules may be extremely high priced as they are no longer manufactured. As a general rule its not worth upgrading any system that doesn't have DDR2 RAM or later as a newer system which will perform miles better can be on par with the price of memory, hard drive and OS upgrades (such systems will have came with Windows XP or earlier which has reached End of Life in April 2014 and should no longer be used).
Once the memory modules arrive you should refer to the Dell Service Manual (older systems) or Dell Owner's Manual (newer systems) which may be found on the Dell FTP website: