I've installed, and now I'm writing from an iMac G4 with 2 GB of RAM (but I've started from 512MB and it was ok) it's a 1,2 Ghz CPU.
Ubuntu 10.04 LT installs really fine with absolute no problems...LiveCD works greats.
Interface is reactive.
I found very big problems using Internet with flash due to here flash is provided by GNU/Gnash and to me let the CPU to 100%.
I think in my case 10.04 is mature for production on iMac G4.
If you can, where is possible, I advice you to compile application trying different performance options and test differences.
You can just directly install the standard Ubuntu release and then install LXDE packages and do the right login on GDM.
PcmanFM as a desktop manager (replacing nautilus) is very light I'm using it everyday, but it's not strictly necessary, you can just switch off the desktop manager and browse files directly with PCmanFM file manager when you need.
You need to focus all resources to the focused application under your eyes (i.e Firefox) using one application at time when is possible.
Firefox is very hungry at all for a 700Mhz CPU (the RAM it's ok), just try to use a tab with gmail and tons of mail in it with a tab of Facebook you'll see that something wrong will be happen near...
Reduce to the minimum the use of swap with the swappiness option...the disk is a very bad bottleneck.
If you can, upgrade the RAM to the maximum possible with the speediest RAM possible, this make the difference...I did it these days upgrading to 2GB...applications thanks for this...today oldest RAM are cheap to buy new...
Do not hesitate to contact me privately for more information about my experience...