SGA size

We have
  W2K server
  Oracle 8.1.7
  30 gig HDD.
  1.5GB ram
  P3 Xenon processor

We want to run 8 instances on this box. They are small databases, some testing, some development, and one for version control. Right now we can only get 7 up and running. The eighth gives us 'ORA_27101 : shared memory realm does not exist'. I am assuming this is due to the fact that we don't have the SGA sized correctly.

How should I set the SGA size for each instance? Are there other parameters I should be looking at. Is this too much for one box?

Thanks

--
Jim Poe (jpoe_at_fulcrumit.com)