Memory for Windows 2000 Server

I have a Gateway 6400 server running Windows 2000 Server and SQL Server 2000 Standard Edition (SP-1). The server has a single Pentium III 1 GHz processor and 256 MB of RAM. There is an 18 GB SCSI drive partitioned into 2 NTFS volumes of 3 GB and 15 GB. Windows 2000 server is on the 3 GB partition while SQL 2000 is on the 18 GB partition.
The clients access seems to be a little sluggish. The 5 client PCs range from Pentium II 400 MHz to 700 MHz Celerons.
The SQL database is about 750 MB insize.
I would like some recomendations on:
How much RAM should this server have?
What should the Virtual Memory settings be?
Is there any tweeking I can do to the server to speed thing up?
Thanks,
Mike Peck