Your system has PC2100 DDR (also called DDR-266) memory, as you noted in your question.
Memory comes in various flavors and speeds. DDR typically maxes out at 333 MHz (PC2700). DDR2 ships in speeds ranging from PC2700 up to PC3200 for laptops.

Laptops use SODIMMs, memory modules that are smaller than those used in desktop PCs. DDR memory typically ships in either a 144-pin SODIMM or a 200-pin SODIMM, but a few systems use something called a DDR Micro DIMM. A DDR2 SODIMM is are also a 200-pin module, but it can't be installed in a DDR socket (and vice versa); each module has a notch in the edge connector that allows it to be installed only in the correct type of socket.

You can identify the type of memory in your computer by using the CPU-Z utility (www.cpuid.com). If you're running DDR memory, there's no way of telling what type of module you have without opening up the system. The situation with DDR2 is a little cleaner, since DDR2 memory for laptops only comes in 200-pin SODIMMs.

For best compatibility, I would suggest getting a pair of 512MB modules and tossing out your 256MB module. Note that if your PC has four memory slots, for best performance you should avoid filling all of them. Usually filling up just two slots is best. If the system has only two slots for RAM, you can fill them both with no problems.