Remember that if you buy a NAS box without drives, don't just put any old drive in the box and expect trouble free performance.
All hard drives are not created equal!
There are special drives that are specifically designed for NAS type services. Unfortunately, suitable NAS drivers are more expensive than desktop drives but they are more robust and have special power features that are needed for NAS boxes.
As indicated earlier in this thread Synology and QNAP are very popular.
Not heard that processors speed is that relevant for file sharing duties.
What would be important in relation to processor and memory is if you wanted to add applications into the NAS (e.g. Bit Torrent, Squeezebox server etc)
Lastly, bandwidth to and from the NAS drive is very different across the ranges so beware that cheaper boxes will have lower i/o bandwitdth.
Generally speaking in the NAS market, you get what you pay for...

Lounge: 861v4+ID40 (hardwired Cat 6), HD621, 2xDSP6000.2VC (l/r), DSP5500.2HC, 2*Jamo ARTina (rears), D2500, 556, 596P, PS3, Humax HDR-1000S
Bed 2: MD600 (hardwired Cat 6) , QNAP 453 Pro