Wireless media server

I'd like to throw together some parts and turn my 200gb USB IDE hard drive into a wireless media server.  Can this be done with a basic old setup (1-1.4ghz P4/Athlon XP, 256 megs DDR, a decent server OS) and a PCI wireless card? 

Anyone have any good FAQs for a complete server n00b? 

Now as for the wireless setup, I already have a wireless router connected to our cable modem.  If I drop in a WLAN card, do I just need to share the folder as >>2 said, or is there anything else I need to do? 

I basically want to set this up as a standalone server that never needs anything done save for powering it on.  I'll probably set up a firewall that denies all Internet access for safety's sake.  I don't want to run AV software or anything so it can constantly just share, share, share.  Is this advisable?