[cernst]

Hot / Cold aisles are a must.

A standard rack depth for traditional servers is 40 - 42 inches. If I were you, leave off the cable management arms...but do have cable management system in the rack!
I am just implementing blade racks and I will not use anything less than 48 inches. Unfortunately our networking team is dictating the use of pass-through network blades on the chassis so they can sniff individual servers...so I can have the potential of 102 copper cables in a rack with just 3 blade chassis....136 with 4 chassis...and then there are fiber storage lines....and power cables.

I've heard that two tiles should be your cold aisle and one tile should be your hot aisle. There are always pros and cons. Your cooling capacitiy and cooling distribution needs to be considered. Opening up a third row of tiles may allow too much air flow and create hot spots may occur in other sides of the data center (I've done that).