Advice on RAID

It really depends on your budget - you can choose between SATA drives (lower cost, higher capacity, lower reliability) and SAS drives (higher cost, lower capacity, higher reliability).

It also depends on the RAID type you're planning, what you want to use it for, and on what the system's purpose is -- you should speak with a Dell server sales engineer before deciding (particularly given that this will be a major purchase in the hundreds if not thousands of dollars).