If I understand correctly, your primary concern is idle power consumption? If so, your gains will be limited with the Intel E5 v2 platform. There's a lot of good advice out there on how to limit power draw under load, but for idle consumption you have fewer options and it looks like most of them are already covered.
A couple things to keep in mind when considering a solution that reuses the DDR3 RAM (so keeping to E5 v2):
- As others have noted: in general, E5 chips from the same generation (Ivy Bridge) will have similar idle power draw at similar base frequencies. The differences are more pronounced under load, but that doesn't seem to be the issue here. CPUs with a lower base frequency will draw less power at idle, but you're going to find the actual savings isn't a whole lot. If you buy a new CPU(s) for a lower base frequency, you'll need to run them for a LONG time before they pay for themselves.
- If you opt for a single CPU Socket 2011 Motherboard (X9SRxx) its going to have the same Intel C6xx family chipset as yourX9DRi-LNF+ board. You'll see some power savings because there's "less mass"in the board to push power through, and if you get one with fewer features (2 onboard NIC instead of 4, etc) you'll see savings there too. But again, you'll find that all else being equal (same CPU, same add in cards, etc.) you aren't going to see much savings over simply pulling one CPU from your current machine. At the savings of only a few watts, it will take you a long time for the investment in a single CPU motherboard to pay off.
- Simply pulling one CPU will give you a noticeable reduction for "free", so you may want to try that just to see how it goes. You note that you are normally peaking at 20% CPU load, so it seems you have some headroom. Depending on your configuration, you won't be able to use all your RAM, but it would be the same case with a single CPU board.
- Switching to a single CPU with higher core count may be an option too. But in this case, you're already at 10 Cores, so you might find the ROI on a 12 core isn't much.
As an example, my system with an X9DRT-F, dual E5-2628L v2, 128MB RAM, and 4x3.5 SATA Drives powered by a Gold PSU in a "desktop" style case drew ~77 Watts at idle with ESXi and no VMs active. Changing to a single E5-2640v2 (but keeping the 128GB of RAM) brought it down to ~55 Watts, some of which was probably from one less CPU cooler and lower overall fan RPM due to less overall heat.