How do you determine which motherboards have lowest power consumption?
Does memory speed affect power usage? In other words will 533Mhz memory use less power than 800mhz and is it a significant difference?
How do I find out which motherboards support VT? (in case I need it later and then I can just change the proc because the E4400 doesn't support it)
Is a target of 50w idle reasonable?
VT-x is on the processor side and most motherboards have it enabled. VT-d is on the chipset/motherboard (at least for Core 2), but that would likely require you to get a workstation or server class board. Sometimes, even if the chipset supports VT-d, it's not enabled in consumer class motherboards. For the most part, VT-x is all you'll need.
Yes, 50W idle is doable with 2x 3.5" drives (green ones preferred). I think a G31 motherboard + Celeron E3x00 or Pentium E5300 (SLGTL) would fit your low idle power requirement quite nicely.
As a point of comparison:
- Case: In-Win BQ656
- PSU: IP-AD80A7-2 (80W unknown efficiency, proprietary form factor)
- MB: Zotac GF9300-G-E
- CPU: Intel Pentium Dual-Core E6300 (Wolfdale)
- HSF: Silverstone NT07-775
- GPU: NVIDIA GeForce 9300 (iGPU)
- RAM: Kingston 2x2GB DDR2 800 CL6
- HDD: Seagate Momentus 5400.5 320GB
- ODD: Pioneer DVR-TS08
- Off: 2W
- Boot: 55W
- Idle: 35W
- Load: 78W
Load is a combination of Intel Burn Test and Furmark. Measured values were before undervolting the CPU by -0.10V (the lowest option available in the BIOS). Load power consumption when undervolted was 72W. I don't think the idle power consumption changed by much. Iirc, the system uses around 40~45W during HD playback.
Add around ~5W for each 3.5" drive at idle and you're still below the 50W requirement. Another factor is the power supply. While I did mention efficiency was unknown for the PSU that came with the case, I'm pretty sure I'm getting fairly decent efficiency in the above build. Since you're targetting 50W idle, skip high wattage PSU's (the extremely efficient Seasonic X-650 being the exception). Ideally, you'd likely want to use a 250W 80Plus PSU. If you already have one on hand, though, just use what you have. Going from 200W to 50W is a fairly significant reduction (~$10/mo at $0.10/kWh) and if you can recoup costs by selling your current parts, then all the better. Quibbling about an extra 10W or so, not really...
By the way, I'm pretty sure the E4400 Allendale is pretty power-efficient already (the M0 stepping, at least). I reckon you wouldn't see drastic power reduction by going from Allendale to low-end Wolfdale. However, the latest crop of Wolfdales also offer VT-x support which is lacking in the E4400.