You could add some radium to make it faster.
Usually for games, the graphics card should be really good to have a reasonable speed. It's much more important than the CPU. But if the computer is made in 2000, then you should also check if you can replace the CPU with a faster one. A lot of things change in technology in 12 years.
At last: The RAM: I think 0.5GB is enough for minecraft, but a bit more won't hurt.