not sure if this is what you mean
If you are using the machine as a server only you should have more than 5Gb
XP will install into about a 1.5Gb partition (2 for comfort)
a second partition 16.5gb should then be available for storage
take the desktop theme back to classic
in performance switch off all effects
shift advanced settings to system processes
leave memory useage as it is
the virtual memory should sort it's self out
if you want a nas box
there are some good linux nas distros around
nut I don't have experience with them
if you just want storage - a usb hdd takes a lot of beating