You certainly should notice a general improvement in computer speed, in particular multi tasking. You should be able to have several applications running at one time and your system should handle the load reasonably well.
However, purse strings allowing, I would upgrade to the full amount your Motherboard allows which is 2GB, and install 2 sticks of 1GB. If that is out of the question, I'd be looking at buying 1GB and leaving one of your original 256MB sticks in to complement your newly acquired 1GB.