First of all, I concur with Amphibians thoughts....they are pretty well on the mark.
Even with 600/700W PSU's in general use these are sitting around 200W or less, even servers with their multiple fans, several hard disks are the same. I further back this up as I monitor the power that my rack utilises and I have a 24 port switch, 5 Active Servers, routers and it consumes 400W. I have a couple of older IBM Servers (3 years old), and even when I turn them on, the total is 750W.
Couple this with 4 other workstations (almost always on), and my Electricity bill is about $400 a quarter, but I do keep an eye on the rest of the house (without going stupid), and turn off lights not in use, turn off tv when not in use etc....
If you haven't already, grab a $25 power monitor that goes in line with your equipment and actually see what you are really using.
I am taking a big guess that you are in Canberra (where they have announced the power price hikes a month or so ago) which is where I live.
However having said all that, I recognise that any saving is a saving.
As long as your Elastix box is a SIP based system only (e.g. no hardware) then virtualising your systems is probably the best way to go....but you do need some reasonable hardware to perform this. When I say reasonable, I am generally talking about a 64Bit processor and utilise VMWARE or XEN.
You can get by with 32Bit processor but you will be limited to version 4 of VMware Server which was the last to use 32Bit processors.
Thankfully due to the products you are using, you do not need large amounts of memory or CPU speed.
I have systems running on VMware and XEN and work very well (production use).
The difference with VMWARE and XEN is that they are installed at the base (e.g. they are not applications that run on a desktop or server). Yes this means wiping the machine that you want to use, but well worth the investment in time.
Just one last note, unless you have done extensive setup in Elastix, I would backup just your Freepbx using the freepbx backup, install the Elastix fresh on your virtualised server and restore the freepbx (I do this on a regular basis with no major issues)....