YUCK! An Emachine!
You have:
Intel Celeron 900 (coppermine) socket 370 100MHz FSB
128MB PC133 and 128MB PC100
mATX form factor
How much you want to upgrade depends entirely on your budget.
There are sever MoBo out there that will fit in your system.
Here are some.
Intel D815EPFVL AOpen MX3SP Chaintech CT-6OIA3T DFI CS32-TL
Anyway Epox, Giga-Byte, and FIC all have offerings as well.
They are boards based on an Intel 815EP chipset and are no longer offered for sale.
It would be a stork of luck if you found one. I think your best bet would be to either just get a PCI Radeon 9000 or start saving for a new to build a whole new system.