If you don't need to cram a lot of boxes into a tight space then 2U will usually give you better airflow and cooling. Also, when those on-motherboard NICs die,

If you don't need to cram a lot of boxes into a tight space then 2U will usually give you better airflow and cooling. Also, when those on-motherboard NICs die, you can more easily/cheaply add additional NIC cards to the 2U boxes. In terms of power, that's actually the main limitation in most colo's these days. Some colos are down at 15 amps per rack, many seem to be in the 20 amps per rack, and some you can get up to 30 but you will pay for that one way or another. Even at 30 amps per rack, for the kind of machines that you're talking about you are still likely to be less than about 20 machines per rack. I just spec'd a build out of the same horsepower & ram but with more juice for e.g., FC cards to a SAN and the safe limit was 16 machines in a 30 amps per rack colo and 8 machines in a 15 amps per rack colo. Shopping around for power information can be tricky because some of the colos like to play marketing games to get you to think there price is better than others but the power charges can eat you alive if you need more than what they allocate in their base charges.