As far as bandwidth is concerned:
PC133 SDRAM = 1064 MB/sec
PC2100 DDR = 2128 MB/sec
As for the amount of RAM, it really depends on what you do to see the benefits. For most people 256MB is more than enough, and 512MB won't really make that much of a difference regardless if it's SDRAM or DDR. However, if you do stuff like video editing then 512MB would definitely help.