You are getting the same amount of 'power' either way (120V x 60A = 240V x 30A), so from an electrical "power" standpoint it is a wash--either one will support the same amount of equipment in the rack...
You should do some research to be certain, but most modern computer equipment is equiped with switching power supplies that accommodate a wide range of voltages (manufacurers don't need to stock a separate power supply for each country; instead, they simply ship the appropriate power cord for the destination country). Given that, your equipment most likely will not care whether they have a 120V or 208V or 240V input--so long as you have the appropriate power cord to connect the system to the power distribution module.
My understanding from engineers at colocation facilities I've worked with is that most switching power supplies are a bit more efficient with a high voltage input (e.g. they produce less heat, which is wasted energy) which can save a bit on both power air conditioning load. The wire gague required to carry the same power at 240V is smaller and more flexible than the heavier wire required at 120V, which may be a convenience when routing cables within the rack.
I would confirm that all of your equipment is capable of accepting high-voltage power; if so, obtain the appropriate power distribution module and cordage to connect your equipment to the 240V 30A circuit.
Remember, too, that a 30A circuit should not be loaded up with 30A of equipment--you need to leave some "head room" to ensure that the circuit is not overloaded (I believe the rule is to stay below 80% of the maximum rated capacity (e.g. maximum 16A continuous load on a 20A circuit; no more than 24A continuous load on a 30A circuit).
On the Dell R710 (and many other makes/models) you can monitor the power usage yourself with this command:
# ipmitool sdr list | grep Watts
System Level | 84 Watts | ok
That's the linux version, but there are Windows equivalents. Graphing that in your favorite tool is left as an exercise for the reader. It should be noted that this gives the power draw into the motherboard. Power supplies are not ever 100% efficient, so add about 15% to that number to get the input into the power supply. Or connect it to a watt meter and measure the efficiency yourself. PSUs are most efficient in the middle of their stated range, somewhere about 50-60% of the rated capacity.
If you are concerned about power usage you might consider using an L-series processor.
What happens when you draw more? That depends on the provider. You'll likely just get a warning (if they even notice at all.) And that's also the scary part. What if everyone draws just over and the circuit breaker trips? How closely do they monitor those circuits? Is it active monitoring or passive monitoring (is there a meter on the circuit or do the building engineers do spot checks with a clamp on meter?) If there is a meter is it per power port or per circuit?
Overall, it's just best to monitor the draw yourself.
How do you know before you order the server? Well, that's a guessing game. Unless you're really cranking on the HW you won't get near the peak.
Best Answer
You can get amp meters to measure the power. or you could take meter readings from your electric meter.
The best way to get good readings is to do this for 24 hours. Not just a few hours. (You mentioned the weekend) and that obviously is a good time. But if you are able to get 2000 servers available for downtime at a weekend, do you need all of those servers?