Depends on a few things. If you are running a 32bit OS your computer will not be able to utilize all 4GB, more like 2.75-3.25 GBs depending on your video card's memory size.
That said, 2 to 4GB of ram is a significant improvement. Just at idle, most computers consume 500MB to 1GB of memory. Launch a few applications and you are already getting close to 2GB. This forces the computer to use the hard drive for additional storage. With 4GB of ram you more then double the effective available memory and will use the hard drive less.