Low-power CPU (and motherboard) question.

I just got a kingston ssd for $110 at frys (2 day sale ended yesterday). It's virtually silent and I like the lack of hd chatter caused by vista or windows 7 in the background. Newegg had an asus itx board with 4 gigs of memory for around $250 in one of their email promos this week. It's one of the few boards with 2 ram slots for using up to 4 gigs of memory. Newegg also has some itx boards with case; the best deal was for $86 recently with free shipping, but they start at $120 now.

Even a low performance SSD would be fine for the OS as it'll do very little with what you want it for, if using Linux/Unix you could probably get away with a USB flash drive for a really cheap option, and the only performance impact would be the boot-up time.

Back to your questions...
I'd probably go for FreeNAS as it's pretty simple but the end result could be pretty much the same with other Linux/Unix.

In addition to the CPU wattage it's also worth looking at the chipset wattage too, as an example on some of the earlier Atom systems the chipset used more power than the cpu.
The AthlonII X2 235e isn't a bad choice for this usage:
- 45W TDP.
- Can use a 700-series chipset which are very efficient.
- Motherboard and CPU are not expensive.
- Since it's fare more powerful than you actually need, you could lower the frequency and voltage (and maybe even one core) to save more energy (leaving the option to increase it again if you ever need more performance in the future.
- Motherboards exist in Mini-ITX, MicroATX and ATX.

Failing that I'd opt for some kind of Atom.