For maximum performance, I like to run my DBs on Hardware RAID Disk Arrays with 0+1 RAID.
NAS - Basically, the same thing as a SAN except it runs over 100BT. I personally don't use them, but I know of several people who do for Oracle DBs.
NFS - As long as your NFS Server is reliable (multi-node failover) you're probably OK. In theory, all that Oracle does is cache data and act like a pseudo-disk farm anyway.
SAN - Depending on your site, may be overkill. Also, while the technology is coming along, it's not really 100% yet (IMHO)
I like to buy disk arrays and tape librarys that are SAN compatible. For example, I buy components that I can create a SAN with one node. Each SAN is an island, they don't interconnect (Yet). That way, I have dedicated disk for each server, but I don't have to worry about security and SAN performance.