I have virtually the same spec pc (Athlon xp2100+)so I guess your is about 6 yrs old too.
Over the past 3 years I have replaced the mx440 graphics card with a vanilla 6600, fitted a better psu, a quieter cpu heatsink / fan, and increased the ram to 1gb.
Nothing else I can sensibly do to it now:
Socket A is obsolete so I can't upgrade the cpu. Adding more ram is quite expensive as DDR2 is all the rage now. A new mobo / cpu / ram bundle is possible, but that would render the agp graphics card useless and I'd need to buy a PCI-express replacement, also I have 2 IDE optical drives and 2 IDE HDD's, but new mobos seem now to come with lots of SATA connectors and only one IDE. And I'm not sure the psu has all the connectors for a PCI-express setup.
My next upgrade will therefore be a new pc base unit, Core 2 Duo, 2 gig ram, 8600gt graphics, Vista