If you have a 1 GB SODIMM in, and an empty socket, then you don't have Dual Channel access anyway. So adding a 256 to it wouldn't cause any additional slowdown - it would still be in Single channel mode, just with 1.25 GB.
Just make sure the 256 is a 667 MHz for the SantaRosa AlumiMac, or an 800 MHz for the new April 08 AlumiMac.
If you put a 667 MHz 256 into this week's new iMac, then it will slow the machine down by dragging the memory bus down to 667 MHz instead of 800.