Depends.
What OS are you using? Provide full PC specifications including detailed specifications on all of the RAM modules, your CPU and motherboard.
If the 256MB module won't run at the frequency of your other RAM, you'll increase the capacity and decrease the throughput.
If you're presently running Dual Channel with your RAM, you will effectively disable that by installing the 256MB module as far as I'm aware, so that's something to consider as well.