actually - to clarify, if you run as a client, the client still does the bulk of the processing (as i understand it), but you'll be upping both the volume of requests and the amount of data transferred, which is why you need a faster, fatter server to do this kind of serving.
ultimately, it's up to you.
If money is no object, get a purpose-built server (like an XServe or whatever).
If time and patience are available and you like fiddling about with innards (and who doesn't), do like ChrisA said, and build yourself one.
If you just want a cheap easy solution for a file server get a LAN enclosure.
If you want a server that you can expand upon, reasonably easy to set up and play with, get an old mac tower that's as fast (both in terms of processor and network) as you can afford. You'll have to balance out the cost of upgrading an older model (adding an ATA-6 card, drives, faster ethernet) against the limitations of what can't be upgraded (motherboard bus speed) against the cost of getting a less-old model. All of this depends a good bit on the pricing gradient for second-hand macs and availability in your area.
ultimately, it's up to you.
If money is no object, get a purpose-built server (like an XServe or whatever).
If time and patience are available and you like fiddling about with innards (and who doesn't), do like ChrisA said, and build yourself one.
If you just want a cheap easy solution for a file server get a LAN enclosure.
If you want a server that you can expand upon, reasonably easy to set up and play with, get an old mac tower that's as fast (both in terms of processor and network) as you can afford. You'll have to balance out the cost of upgrading an older model (adding an ATA-6 card, drives, faster ethernet) against the limitations of what can't be upgraded (motherboard bus speed) against the cost of getting a less-old model. All of this depends a good bit on the pricing gradient for second-hand macs and availability in your area.