Have a client network that has around 5 macs with only about 3 ever in use at any one time we are replacing their aging G5 mac pro with a new intel mac pro.  They currently only use the existing machine as a glorified file server.  So getting to the point when does one need to buy Mac OS X Server license vs just using regular mac OS x to share files?

We tried to use both a Linux box with nfs, samba, apple file sharing instead but unfortunately the mac client desktop's have files open in Adobe Creative Suite and often rename a folder or even the open file without closing the open files and this causes problems when using a Linux or Windows server (it seems I can get the linux box to work okay with renaming open files/folders in Photoshop but not in-design or illustrator).