If you're pinching the dollars, there is little better you can do than save money and build it yourself (given that there is no extra labor cost incurred for doing so). Many shops will also assemble a system for ~$250-500 and another $250-1500 for a 3-5 year warranty (depending on the cost of the system etc). You could also go with IBM/HP/Oracle and lease the bunch. It's really up to you what type of support contract you require (generally you won't need it). You could calculate the Amazon/"shared hosting" aka cloud route but I figured out (in my instance) that unless you need the thing less than ~30% of the time in a year, you are better out doing it yourself.
I am currently running several SuperMicro GPU workstations (4U with 4 GPU's each) and several more SuperMicro 1U/2U servers (storage). They are run-of-the-mill and really nothing anyone can improve upon, especially cost (and I've had several salesmen concede that point, most recently Nexenta). Off course if you don't know anything about it, it might be worth having a support/management contract.
Also a rack drawing 25kW peak does not mean you require 25kW of power. IF you run at more than 70% of your peak power as rated by the power supplies, you are running dangerously close to the limits. I run a half rack of storage and a GPU server and it draws a consistent 20-30A@230V (that's 4-6kW) although it's peak power based on power supply alone is closer to 15. The power supplies are really intended for the worst-case scenarios (8x 3.5" HDD, 100% CPU usage, 100% RAM slots filled, 4 5.25" devices, 4 GPU and several other add-ons).
As far as Xeon/Opteron, you would really have to benchmark them both against your application. For some loads one outperforms the other and although AMD is generally cheaper per performance unit, Intel may allow you to get more out of a single machine.