Detail on RAC with topic available on these link
Competitive and Business Information
There are tons of links on Oracle's site for reasons to do RAC. Just remember, that Oracle wants to sell more licenses.
RAC (IMprofessionalO) is for high-availability and/or heavy-workload systems. I have had 3-node Sun 6900s (48xDualCore x 192G memory 8 HBA's 8 NICS and >300TB of storage. I have also configured 2-node RHEL 4xdualCore x 64G x < 1TB storage. Each with it's own unique requirements. I am a fan of ASM. In most cases with data-center level SAN storage systems, you can expect a 10-20% performance improvement just by migrating from filesystem to ASM with the added expense of RAC. If you do RAC, then do ASM.
So the question to you is: Why are you considering RAC? What is the business driver for RAC?