Error installing RAM

So sorry is this out of the forum''s topic, but I don''t know where to seek help. Short version: I just installed a DIMM SDRAM of 256M. When I start the computer, I could detect the memory, thus I have a total of 380M of memory. However, the system performance and resources available did not improve at all. The 256M somehow could not be used even though it is detected. More Detail: I did not receive any error message indicating anything wrong with the RAM. I took the new RAM piece to where I bought and ask for them to do a RAM check and they said there''s nothing wrong with it. Since my cousin also has an HP, I tried installing it in his computer, still no effect. I tried installing the new RAM alone, it serves as much purpose as a 128M. I tried installing two 128M, still no effect. When I run large graphic programs or multiple of programs, I run out of resources or the programs run very slowly both when there''s only a 128M installed and when I combine the 128M and the 256M. In other words, the performance is the same whether or not any additional memory is installed. What could be wrong?