Processor: 1.8Ghz P-4
Mother board: Intel 845
An Oracle database requires more than just physical disk space; it also requires system resources and memory. From the specifications of your server, I would say that the limiting factor to how many databases you can create is the memory available, not the physical disk available. It also depends on the release of Oracle your databases will be, and how you configure the SGA (system global area) since this impacts memory usage the most. With only 512 MB of memory available, I would guess that you might be able to squeeze two Oracle databases (with small SGA sizes) on this server.
For more information on what to consider when creating a database, refer to the appropriate administrator's guides. If you are planning to create Oracle9i databases, refer to the Oracle9i Release 2 (9.2) Database Administrator's Guide. For creating Oracle 10g databases, the Oracle 10g Administrator's Guide.
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