A cheap DP upgrade would be a Dual G4-500 just make sure you get the correct heatsink and your G4 system is ver 7 to be able to run dual CPU's. I have one setup like that and just got another in today (ebay $20 including shipping just snagged it because I noticed the dual processor heatsink).

If you want a decent G4 system a MDD is cheaper then just a fast CPU for a Sawtooth (or get a Quicksilver).