When you have to choose between server or PC, It depends upon your requirement and budget. As you said you are using pentium III machine PC since long, I suggest not to go for server. Quad core PC is enough with 2 to 4 GB of memory for more then 100 users.
You have also asked for SATA or RAID. RAID is a fail over mechanism of HDD where SATA is HDD. You can configure SATA RAID to to save your data when any of the HDD goes crash. It is the best option when you configuring the server. But as I suggest you to go for PC, you have to buy SATA RAID controller card or Motherboard with it.