DDR, or Double Data Rate, is almost twice as fast, and yes, if your MOBO supports Dual Channel Access, two sticks (if they are a matched pair) will allow the Dual channel access to function, making it even faster.
LV38_Eagle, brother, you speak the truth. However, if you use 2 sticks instead of one, you are using up more ram slots as opposed to a 1-stick setup, which can make future upgrading harder and possibly more expensive since most (newer) motherboards support 1.5-2 GB (3GB if you run a server platform!) of memory, but only 3-4 slots. It all depends what is more important to you and your specific needs.
And yes, DDR memory is about twice as fast as SDR, and isn't even that much more expensive!
DDR, or Double Data Rate, is almost twice as fast, and yes, if your MOBO supports Dual Channel Access, two sticks (if they are a matched pair) will allow the Dual channel access to function, making it even faster.