Agree with Hemant about benefits of RAC, can only add that drawbacks are that if you place badly written application on the RAC the performance may get worst. So the application should be designed for RAC. Its good idea to check other users experience of you application running on RAC (I dont have experience with PeopleSoft)
Also pls check articles on oracle-base, they are very good written and very helpful
Dataguard is guarding the data from disasters by placing it in different server/storage far from the production.
RAC is the cluster solution form oracle for which is used to connect a database from different nodes (load balancing and scale up with out down time)