Since your MB is only 266 fsb and supports both sdram and ddr it sounds like one of the older ali or SIS chipsets.
The difference in memory bandwandth between sdram and ddr is double at least more if the sdram was 100. How much of that you will see depends on what you use the computer for.
Gaming - you'll see some difference but it depends on the video card. If card has enough memory to store all the textures in the vid card the improvement will be low.
Office applications - You won't see much here either. The 1 gig RAM is already buffering the applications and data.
Video or photo editing, file compression, etc- you will see the most improvement.
Just going from sdram to ddr is not goign to give you a big bost in most cases. Spending the money on a video upgrade may be a better investment.
Post your full system specs. vid, mb chipset, proc type P$, AMD, etc.