What operating system (most likely some flavor of Windows) is the system using? By the way, the max it supports is 384MB. As this is quite an old system, there really isn't much you can do to make it faster. Adding that last 128MB of RAM will cost more than it is worth and it will do very little to help speed things up.
I have a similar Sony sitting right behind me. Nice "old" system, but very limited by today's standards.