G'day and welcome to the Forums.
Firstly yes you can upgrade a Sawtooth, Gigabit Ethernet, Digital Audio or Quicksilver model with that CPU upgrade. There are quite a few brands on the market so do your research prior to rushing in. Sometimes OWC, a Mac supplier, has specials on their upgrades and personally believe their upgrades with 2MB L3 cache are faster.
Suggest you look for a Quicksilver machine. They have a 133MHz system bus speed , faster than the earlier models which are only 100MHz, and have a 4X AGP slot compared to a 2X.
Max out the RAM to 1.5GB, 3 x 512MB PC 133 Apple compatible low density RAM. Generally the PC RAM on eBay will not work. After that,. if thew readies are holding out, look for a 7200 RPM 16MB cache HDD and a faster graphics card.