hi,
as u might be knowing pretty well., a hybrid database breeds more problems then u would like to expect and mostly systems which have this combn. resort to OLTP in the day time (peak usage) and DSS in the night time, this essentially is maintained by using two sets of configuration files, but as usual oracle provides and takes care of these issues in a default way once u install uer oracle server, once u have made up uer database using the default then based on its usage for the next say 3-6months u should be able to tell more about uer db., i for one would prefer to size the sga that way,u would be going into perf. tuning this way and the only way to tune is the iterative way
cheers
soren