P35+E8500 = Factor in about 150w
16 port RAID card = Factor in about 10w (for the card only)
DVD drive + O/S drive (I'm assuming you'll have one separate from the 20 1TB drives..
) = Factor in about 25w
Miscellaneous things (like KB/Mouse/Serial port/etc) = Factor in another 10w
20 x 1TB = Factor in about 400w (20x20) max. However, unless your RAID card supports staggered spin up, you need a PSU with atleast 65-70 amps on the +12v rail (20x3A = 60Amps + 5-10Amps for the CPU and motherboard startup)
All said, you need about 600-700w, BUT you need to size the Amps accordingly, if your raid card does/does not support staggered spin up.