I don't think WOL would get the machine running soon enough for use as a file server and print server. I believe the solution is just to intelligently spec the machine to use as little power as possible from the start.
If you're serious about having the ultimate low power system and don't need a super high performance file server (RAID on GBe) then get something like the following:
AC adaptor
Pico PSU claims near 100% efficient
a VIA mini-ITX board
Combine those parts with a USB printer and a single, large hard drive and you'll have probably the most power and space efficient server you can imagine. I'd be willing to assume that such a setup will probably consume less than 50 watts idle - and that's including the printer. Note that these parts are also available elsewhere, I just linked to this store because it was convenient.
If you want to stick with more "common" parts:
Sempron 3000+ (supposedly lowest socket 754 with Cool and Quiet)
SiS chipset board with IGP (I believe uses less power than Geforce6100/6150)
Use the highest efficiency and lowest power output PSU you can find because PSUs usually are most efficient closer to the maximum output. Inefficiency is just wasted Watts. Let's say a 400W PSU is 70% efficient at full draw, meaning when you have components drawing a total of 400W power, the PSU actually draws around 571W from your wall outlet and wasts the extra 171W as heat. The same 400W PSU may only be 50% efficient at a 200W output, meaning if your components draw 200W, the PSU takes 400W from the wall and wastes the extra as heat. Thus the importance of high efficiency and as low an output as you can get away with while being stable.
Use integrated graphics only, and one of the lower performing offerings instead of the highest. That graphics performance won't help a server at all and just makes the system draw more power.
The lowest CPU with Cool and Quiet - I've heard that it is the Sempron 3000+ for socket 754. The reason for this choice is that a lower active transistor count will equal lower power usage and the 128k cache Semprons are the lowest on that platform, plus with Cool and Quiet, just get a third party tool such as RMClock and manually set the CPU to the lowest multiplier, then set the voltage to the lowest that it can run stable at that speed. I do something similar on my Pentium M notebook (using CrystalCPU) and my battery life increases dramatically while still being powerful enough to watch movies for 4-5 hours on a charge (on long flights).
Minimize the number of hard drives. Each hard drive adds between 5-10W of power draw. Go for a single 500GB drive if you need the space instead of three 160GB drives, for instance.
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