What is the model of your optiplex?
You can try to obtain a 2GB stick but to avoid problems it should be the same type and make as the existing one. Better still buy a new matching pair of 2GB sticks and discard the existing one.
As your OS is 64 bit you might consider a matching pair of 4GB sticks giving you a total of 8GB provided your Motherboard will take it. Without the model number I cant help further.