I did some stats on this a while ago FWIW, using the handy dandy kill-a-watt..
Typical Developer Dell PC
(2.13 GHz Core 2 Duo, 2 GB RAM, 10k RPM 74 GB main hard drive, 7200 RPM 500 GB data drive, Radeon X1550 video)
Sleep 1 w
Idle 80 w
One CPU core fully loaded 108 w
Both CPU cores fully loaded 122 w
Standard Developer Thinkpad T-60 Laptop
(core 2 duo 2.0 GHz, 100GB hdd, ATI X1400 video)
Sleep 1 w
Idle 66 w
One CPU core fully loaded 74 w
Both CPU cores fully loaded 82 w
LCDs
Old Dell 19" 50 w
Ancient, giant 21" NEC 67 w
New Dell 19" 28 w
New Samsung 19" 28 w
Apple 23" LCD 72 w
Samsung 24" LCD 54 w
It turns out with the LCDs the default brightness level has a lot to do with how much power they draw. I almost immediately turn any LCD I own down to 50% bright, just because my eyes are overwhelmed if I don't...
Your figures sound like they're on the right track in general, a dual disk, dual processor box like a HP DL380 pulls about 350-450w on average so add in 43 x 15-20w for the disk and you're not far off a Kw per chassis. This gives you two problems, you're going to need 4 x 32A PDUs to support dual PSUs and you've got a reasonable amount of heat coming from the rear if the rack that needs scrubbing. I also suspect that your hosting company might bitch about this and insist on you splitting your kit between 2 or more racks instead.
That said I can't think of any other way of getting that amount of disks into that density, what make/model are the 3U boxes by the way?
Edit - have you done the weight calculation yet? You should run that by them too, that sucker's going to be quite heavy.
Best Answer
You'll never find the exact numbers on a sales website, because it depends entirely on how you configure your server.
What you can find however is the wattage of the installed power supplies. For normal rack servers, the power supplies are 1+1, which means that the power draw will never exceed the total of one power supply (so, 2x 850 watts is still just 850 watts). Blade servers are a different story (as they have 6 power supplies).
For the ubiquitous Dell R710, it has the option of 570w or 870w power supplies (in my region at 240v). 0.5kw is only 500w, so you could really only get one server at peak load before going over your power limit.
We have some R710's though and I can tell you that at 240v with 48Gb of RAM, 2x 6-core Xeons and 4x 15,000 RPM SAS drives they draw no more than about 300w under peak load. That's measured by the UPS and confirmed by the iDrac logs. So you might be able to get two of them under normal load into 500w (this is based on the assumption that your rack provider will simply bill you exorbitant amounts based on excess usage. If you're going to trip a breaker by exceeding your allocated power usage, don't take any risks).