Optimal disk config for database server

Hi,
I am hoping to get some answers and ideas from you guys regarding on what to choose.

What to think of when purchasing disks to a database server?
Scenario:
Required storage: 600GB – 1.4 TB (depending on installation size) // DB design requires quite a lot of data to be transferred between DBs and tables
What disk speed should be used? (10K rpm or 15K rpm?)
RAID solution? What to prefer to ensure optimal performance.

To go further.. should I mix RAID config and sets to put temp db on single physical disk?
i.e. RAID5 for customer DB and RAID 1+0 for TEMP DB.

What are your thoughts?

Regards
Nicolaj Aaroe