yes and no. you do have SDRAM, but not just plain SDRAM.
your mobo supports DDR400 (PC3200) DDR-SDRAM.
what I couldn't figure out however is if that via chipset supports dual channel or not.
why do you feel you need the additional 512MB anyways? and how did you pick that number?
while it is not always required, it is always recommended that you use matched sticks of RAM. you don't want to mix different RAM sticks (they may work fine, but many times cause errors or crashes, and it may even refuse to boot)
if you dont do any gaming or video editing, then 512MB is all you need, so in that case another 256MB (the same as your current stick) will be sufficient.