My PC draws about 200 watts AC under non-gaming conditions. This is determined by the power meter on my UPS. Granted this includes a couple of other components attached to the UPS as well, including a very power hungry display.
The power efficiency of the Sandybridge-E processors is no worse than the Sandybridge processors. Under idle conditions they will idle at 1200Mhz rather than 1600Mhz which is what the quads idle at.
One thing that the RIVE has going for it that is not common on other X79 motherboards is that it will apply the top single-core turbo bin to all cores automatically. This turns the 3/3/4/5/6/6 turbo scheme into a 6/6/6/6/6/6 scheme without any changes on part of the user. You can of course simply pick a higher turbo multiplier without changing much else, if you want 8/8/8/8/8/8 it's just a single field in the EFI setup.
As for memory, I'm currently using two 16GiB sets of Mushkin Enhanced Redline DDR3-2133 (9-11-10-28) SDRAM. DDR3-2133 puts a massive load on the IMC and I wouldn't recommend something that fast in 64GiB, however there are 32GiB sets of Mushkin Blackline. Blackline is slower and conforms to the JEDEC standards, where as Redline is performance optimized and non-standard.
http://www.newegg.com/Product/Product.aspx?Item=N82E16820226338
You should be able to put two of these in there easily enough.
One thing that is very important, and I'm going to put this in bold because it's easily overlooked:
When installing more than 4 UDIMMS it is very much recommended to attach the EZPLUG2 auxiliary power connector to ensure stable current delivery to the UDIMMs. Most X79 boards have at least one auxiliary power connector in addition to the standard 4 and 8 pin EPS12v connectors and 24 pin ATX connector. These additional auxiliary connectors usually come in the form of a 6 pin PCIe connector for additional PCIe slot power (only really needed for 3 or more power hungry PCIe cards) and either a 4 pin MOLEX connector or a 4 pin floppy connector. The 4 pin connector may also power a few other things too such as some of the onboard USB headers but I need to experiment with this some more in my spare time. Thus, there are a total of 5 power connections from the PSU to the motherboard alone!