i have a pc that does the same as yours, think its only got 256mb in it, if you have antivirus on it, then set it to ignore the media filetypes.. eg avi, mpg, mpeg, mp3, vob, jpeg and you'll stop it scanning things that you know are fine
works fine sharing pics/video/music between 2pc's and 4 macs, its a p4 2.8 i think, its on a 100mbit nic to the rest of the boys, and i have a direct gigabit connection to it (as i administer it!)
also, set it to read only for all of the media, i remote desktop to the server and copy things across using the server to control it (so no one, not even me can edit files across the share) easy to setup, simple file sharing works fine!
-just remoted in to it and it does only have 256mb, one thing that i do is not share anything on the same drive as xp runs on (well a couple of small files is ok), just to keep the os running smooth, not had any problems at all since i set it up a few months ago