Processor Upgrade

My computer has an AMD-KG-Z @350MHZ.
What should I buy to have a great upgrade.
OR should I buy a new computer.
I have changed a few things in the tower , I am not afraid to go in there, like the memory & drive g. I am willing to change the drive & the processor and the OS. I just do not want to do do something stupid.
Advice most appreciated !
Ken Clark