When you say 20-40W idle are you guesstimating that I'd see 20-40W difference in idle wattage between the L5630 and X5650 or are you saying that the X5650 likely consumes 20-40W idle? And is that per each or including both processors? Sorry, probably stupid questions but I have no idea how much power CPUs consume when idle. I guess I was hoping it was quite a bit less than they use at 100% (I know the newer chips are a lot better in that regard).
From memory (I don't have my notes in front of me) from a very recent test comprising of ..
Supermicro 823TQ-650LPB chassis (I believe the PSU is 80plus efficient)
Tyan S7012 DP 1366 motherboard (has quad NICs standard)
6x Micron 4GB PC3-10600R memory
2x case fans operational (other 2 disconnected)
2x Intel STS100C active CPU coolers
1x Seagate 160G SATA drive
Server 2012R2, OS booted and idle after login
Using a Kill-A-Watt to measure wall plug draw ..
With 2x L5639's the system idles at about 125W
With 2x 5650's the system idles at about 130W
So very little idle power difference between the 2.
Now, the UP and DP 1366 motherboards typically implement the Intel 5520 (IOH) Chipset, supporting 36 pci-e lanes. I know from experience that the 5520 runs hot and needs to be well cooled.
The Supermicro X8DTH-6F that you have purchased is an I/O monster, and has 2x 5520 IOH's on board as well as al LSI SAS controller, so I would expect idle power consumption to be higher than the figures I have mentioned here. (The Tyan S7012 only has one 5520 IOH)
What would I be sacrificing though, besides losing the extra 4-6 cores? Aren't certain PCI slots connected directly to one IOH or the other? I assume I lose the other six memory slots (not really a problem at the moment since I've got 6x8GB DIMMs).
I don't think you will lose anything running the X8DTH-6F with only 1 CPU. Clearly you only get access to the 6 memory slots that are hard wired to the CPU slot. However, the dual 5520 IOH's are connected with QPI and "present" themselves as a single IOH to the CPU(s), so even though 4x pci-e slots are hard wired to one 5520 and 3x pci-e slots to the other, you still have the ability to connect to all 7 pci-e slots from one CPU.
This certainly changed with LGA2011 designs where the pci-e lanes are driven directly from the CPU, and in DP 2011 designs the pci-e slots are hardwired to specific CPU's