To fragment or not to fragment - opinions requested

On a SMP (4 processors) system is it better to fragment all of the tables using round robin, or should the tables be put is different or the same Dbspace? My main concern is for performance on a database that is normalized and uses several joins on queries. Yes we will be changing the design to de-normalize some of the larger tables as new software is developed. Currently the database has about 150 million rows and is growing. An input on the trade offs of using fragmentation or not will be appreciated. Bob Informix version 7.10