Truth be told any money spent on upgrades for that old Socket 478 system is wasted, as there will be nothing of it that can be moved over to a new computer if and when you get one. It just doesn't make much sense to spend money on something with limited or no support for a new operating system.
In your case,
1. CPU. Socket 478 is pretty much dead and buried, and for the most part any "new" cpu's for it will be used (Newegg for example lists all of one socket 478 cpu).
2. Motherboard. In addition to being socket 478 it is using a SiS based chipset that hasn't seen a driver
update in close to 5 years, primarily the AGP/GART driver which is needed in order for an AGP card to work properly.
All things considering you would be far better served by saving up and replacing the entire computer.