One of the problems when judging the "performance" of a machine is just how subjective it is. Is the problem that the GUI 'feels sluggish', or that the machine actually requires a lot of "wall-clock time" to do certain tasks, like opening large files or rendering a webpage? There are limits to how much magic you can expect from a RAM upgrade.
My benchmark for 10.4's performance on a slow machine is my old B&W with 576MB of RAM. Honestly, for the sort of tasks that machine is reasonably capable of performing that amount of RAM seems to be adequate. I did have it bumped to 1GB briefly (I removed it when I determined that the new RAM wasn't entirely stable), but I don't think it performed any better from a subjective standpoint. (Maybe if I'd had a lot of things running at once? But on such a slow machine I wasn't likely to be tempted to do that...) Swapping the G3 ZIF for a G4 ZIF (at the same speed, 400mhz) actually *did* result in a genuinely noticeable improvement in subjective speed, however. OS X's eye candy *really likes* AltiVec. Wall-clock time for long-running tasks didn't change much (and the machine benchmarks essentially the same as before when running non-AltiVec-aware code), but the GUI speed did pick up quite a lot. iTunes visualizations, the photo-scaling screensaver, dock minimization/restore animations, all went from "obviously slow and hitchy" to "Smooth". Maybe a little leisurely, but smooth.
Note by saying this I'm not recommending you spend the money for a G4 upgrade on a Pismo. For a machine that old it's good money after bad, and you could probably find a working PowerBook G4 for less than what the card costs that would run rings around it. And any G4 Powerbook will, with the possible exception of the very first 400/500mhz models that share the same non-Quartz Extreme-capable video chip. (Of course, those things were so fragile you probably won't have to worry about being tempted by one.)
My benchmark for 10.4's performance on a slow machine is my old B&W with 576MB of RAM. Honestly, for the sort of tasks that machine is reasonably capable of performing that amount of RAM seems to be adequate. I did have it bumped to 1GB briefly (I removed it when I determined that the new RAM wasn't entirely stable), but I don't think it performed any better from a subjective standpoint. (Maybe if I'd had a lot of things running at once? But on such a slow machine I wasn't likely to be tempted to do that...) Swapping the G3 ZIF for a G4 ZIF (at the same speed, 400mhz) actually *did* result in a genuinely noticeable improvement in subjective speed, however. OS X's eye candy *really likes* AltiVec. Wall-clock time for long-running tasks didn't change much (and the machine benchmarks essentially the same as before when running non-AltiVec-aware code), but the GUI speed did pick up quite a lot. iTunes visualizations, the photo-scaling screensaver, dock minimization/restore animations, all went from "obviously slow and hitchy" to "Smooth". Maybe a little leisurely, but smooth.
Note by saying this I'm not recommending you spend the money for a G4 upgrade on a Pismo. For a machine that old it's good money after bad, and you could probably find a working PowerBook G4 for less than what the card costs that would run rings around it. And any G4 Powerbook will, with the possible exception of the very first 400/500mhz models that share the same non-Quartz Extreme-capable video chip. (Of course, those things were so fragile you probably won't have to worry about being tempted by one.)