it says your cart is empty. but really though, your current system isnt very upgradable as it is... the better option, would be to keep your system how it is, and save up enough money to assemble 'most' of a tower from scratch, and reuse parts from your existing system when possible to fill in the unpurchased (needed) parts.
a retail copy of the OS you know youll need
if you want to reuse your existing ddr1 memory, youll need an ATX or mATX motherboard that has ddr1 memory slots (ddr2 has 240pins, wheras ddr1 has 184pins, so physically theyre not compatible)
a socket 939 setup is a fairly inexpensive option, making it possibly your best choice currently, if youre wanting to keep your memory anyhow (and soon to be cheaper in the next couple months or so i believe, due to another price drop from amd)
socket 939 supports either agp or PCI-e, some motherboard have both (and some mATX motherboards have an integrated gpu too, giving you the option to upgrade to an add on gpu later on, integrated gpu performance typically isnt top notch either though)
youll need a new cpu, preferrably a s939 X2 or a s939 dual core opteron if you can
youll need an ATX power supply (preferrably a seperate purchase), with the needed amps dependant on what hardware you purchase
youll need an ATX case (without power supply), something as cheap as $40 may work here
and, you can possibly just reuse your hard drives, cd drives, floppy drive, monitor, kb, mouse, speakers, etc
if you were to ditch your ddr1 memory though, and go with ddr2, youll have more upgrade options available