that system you should better leave it as is.
probably none of the new guys took a good look at your system, but to clear it also up for them:
-133mhz fsb with his SIS chipset...
-133mhz fsb northwood P4A (@ celeron speeds...)
-2x256mb single channel RAM and both slots filled up (possibly even pc2100!).
gaz, keep this system like that, because any upgrade you can possibly make to it is by far obsolete for this 4year old system (motherboard dates 2002!!!)
save up and get something with some performance potential.
i.e. Athlon64 /IntelPentiumD920/805