Definitely put all disks in a single RAID set for better performance. Once loaded, ESXi runs in memory and only writes logs and configuration data. The disk space required for ESXi - including a 4GB scratch partition - is ~5GB. In ESXi versions prior to 5.x it was a good idea to place ESXi on a separate disk in order to be able to reinstall the system, but with v5.x ESXi doesn't reformat the complete installation disk anymore.
Even if you still want to separate the OS from the data, you can still create a RAID set with all disks and - using the RAID controller - create logical volumes on this RAID set (e.g 10GB for ESXi).
André