What power supply do you have? If you go with an AMD processor, you may also need a bigger power supply too. If thats the case, your going to spend even more money on a motherboard, AMD processor, heat sink-fan for processor, and a power supply.
If money is a limit, which it is for most of us, I'd go with a 1Ghz CeleronII chip and a slot to chip adapter card. Get a name brand adapter card that supports the CeleronII copper chip, then overclock it and try to get to 133Mhz frontside bus speed with it. This will require 133Mhz memory. You'll get the most out of the parts you have without spending a lot of money. Past this or best bet would be to sell what you have and build a new system because you'll be close to that point anyways.