i'm using the amd athlon 2 dual core 3.0ghz chip. although it is not screamer compared the the insanity of some people's processors, it is more than adequate for me. basically, i have that chip and the onboard video (radeon 4290) and i was absolutely amazed at how fast my pc runs. the only thing it's a bit slow with is ripping dvds. now granted i'm not playing the absolute most cutting edge games, but return to castle wolfenstein and doom 3 play flawlessly with all the setting turned all the way up and the best screen resolution that my 23" monitor will allow.
so, my opinion is that the dual core athlon 2s will be more than what you need for building a file server.
this is the processor i'm using http://www.newegg.com/Product/Produc...82E16819103903
it appears that the extra cores are not unlockable on these chips, but i think there are some athlon 2s and some phenom 2s that are. so you might look into that and get a dual core chip that has unlockable cores and then just unlock em for some cheap quad core goodness.