A 32 bit OS (XP, Vista, Win 7, etc) can only access a little under 4GB of RAM. I would upgrade to 4GB, if the PC is capable. You will have somewhere around 3.2 to 3.5 GB of RAM available (the rest is used by the system and video).
A 64 bit OS will address more than 4GB.
If you post the Dell model, I can check exactly what is needed. Or, go to www.crucial.com
and run the configuration there and it will show you what is needed. I used to do a lot of support on the Dell forums and we always recommended Crucial for non-Dell memory as Crucial is guaranteed to work in Dell's. Many Dell's have a history of incompatibility with memory, some brands will work in one but not another. Do not use the Kingston Value Ram (KVR) as in almost all cases it will not work or will be flakey in a Dell.