Is this a desktop or laptop system? In a laptop I can understand you have a 2GB and a 1GB SODIMM. In a desktop that would be unusual, and possibly a severe limitation. On a desktop, going to 2 x 2GB (preferably matched) would be a very good idea.
As a general answer, more memory is often a good thing, within limits. 4GB of RAM for Windows 7 64 bit is a good idea. Going above 8GB, not so much.