performance benefits from distributing across disks

I am interested in finding out how much of an improvement I might expect by distributing the I/O more evenly over more disks and by splitting the indexes and data into separate tablespaces (on separate disks). I am planning on doing this in the near future but I would like to have some idea of the amount of improvement I might be able to get. Currently, iostat doesn't show a very heavy load on the disk (at least not continuous). I am mostly interested in improving response times since our volume isn't very high. Will distributing i/o improve performance even when the load on the disks is light?

In a related question....if the indexes and data are on separate disks, is Oracle able to request both sets of data at the same time or does Oracle request the index and then the data? I sort of figure the latter and that the performance improvement from splitting across disks is to allow multiple sessions to access data at the same time (when they're not on the same disk). In other words, one user can retrieve index data while the other retrieves the actual data thereby reducing contention for the disks.

     I would appreciate any insight/experiences with the performance benefits of distributing the data across disks.

        thanks,
         jim