I simply use a modest-spec PC to handle all my file sharing needs at home and have a similar set up to that mentioned by iscatel albeit I don't use Webmin and prefer Fedora Core 5.

A dedicated server would be preferable, but you're looking at double the cost or more to set things up, and unless you have mission-critical data on your machines that needs to be stored securely and backed up daily, purchasing a server may be a little over the top. Instead, you could simply purchase a low-end PC and equip it with either wireless or Gigabit ethernet if network speed is a big issue for you and also fit additional hard drives and a RAID controller. That will take care of storage as well as backup facilities. Unless many machines will be accessing your server simultaneously, I wouldn't worry too much about getting a high end processor or kitting the machine out with several gigabytes of RAM as you don't really need it from what I can tell.

If you want fast network performance, wired is still the route to go. Gigabit ethernet isn't ridiculously expensive anymore and is about as quick as you're going to get in a home environment. Though wireless is improving, it can still be slow progress transferring gigabytes of files at a time.

That said, if you can afford a standalone, ready to go server solution then go right ahead, though personally I would recommend using either a low-end server if you can get a good deal or consider using a desktop machine albeit kitted out with one or two extras to work as an effective network server.