Oracle offers the RAC Real Application Cluster Solution based on shared everything architecture.
within a cluster you have Oracle Instances sharing the datafiles from spfile, controlfiles, to tablespace-datasets ..
the benefits are smooth failover and scaling ; though you can add some more server to your architecture instead of buying new bigger iron.
Mike Ault's book on Oracle 9i RAC gives a good foundation and background and different architectures in its first 300 pages and in the following pages how to set it up an make it running.
But keep in mind Oracle is still improving their step by step implementation guides and they can be retrieved from metalink.oracle.com and their are a couple of pitfalls and even nightmeres whith some specific hardware, firmware, OSs and RAC implentations on it .
at least you need two server each running an Oracle Instance an shared disks (perferable SAN Storage ) with fibrechannel connections and another connection for the heartbeat, quorum raw devices and either raw devices or set up the OCFS Oracle Clustered File System ...
Actually I'm writing a cookbook from my experience but have not finished it yet ...