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?