Or should I just buy a new paired set of 2 x 2 GB RAM to have 4 GB?
It looks like you have a DDR2-667 module. You could buy another identical module(CPU-Z can be used to view the part number of the current module) or a new 4 GB kit. If you buy a new kit, look for DDR2-800 modules that require 1.8V. Either way, they'll run in dual channel mode if installed in the same color slots.