Go to the dell website, look up your model.
OR (even better) go to a memory manfactuer website like crucuial, kingston, corsair, and download their online programs to scan your computer. (They may ask you to input the make and model.) The crucial site has a scanner program. Unfortunately you have to use internet explorer.
It is imperitive that you use the correct memory type for your system. Windows XP is a memory hog and uses about 200MB just for itself. The minimum memory for most computers today that is acceptable is 512MB. I recommend more. I have 1 1/2 gigs of memory in my system.
Memory is about one of the most cost effective ways to speed up your system. Always double or triple what you think you'll need. Always use matched memory (that is the same make and model for each type in your system.) If you don't, you can run into problems. example: all corsair, all kingston, or all crucial.
Stick with a tier 1 type memory (like the brands I mentioned.) Using a generic type and you may run into problems as their bandwidths tend to be inferior. (You get what you pay for.)
Most motherboards have a tech book (RTFM.) - it will tell you what brands are compatible with your CPU. Otherwise get the info from Dell.