I have been there and done that. One thing you should at least consider is seeing if you can get some of your more creaky servers to run in a VM, you can run DO

I have been there and done that. One thing you should at least consider is seeing if you can get some of your more creaky servers to run in a VM, you can run DOS in virtualbox and probably xen as well, you might be able to replace a number of physical machines with a single virtualisation box ( a pair if you are serious about availability ). Going this route should save you energy and space. If you can replace a rack full of machinery with 1U of server and 2U of disk shelf, you are way ahead of the game.