These are the cpu's that you can put into your existing motherboard click here. Though they will be hard to track down. Your best bet would be second hand.
Your motherboard can take pc2700 ( 333mhz ) and pc3200 ( 400Mhz ). You can use all three slots with pc2700, though only two slots work with pc3200. You could have a maximum of 3Gb of ram in this motherboard.
Unfortunately if you were to go for another motherboard, the cpu, memory would probably be obsolete. So you would have to add the cost of those on top. Also you will find new motherboards that support AGP, though you would be better trying to find one that supports agp and pci-e as well. So if you do upgrade the graphics card in time, you can go for the newer standard of card.