I recently installed Leopard on my 1.33 Ghz iBook and my 933 Quicksilver. Both seem to run just as fast as Tiger did.
Unfortunately, with the release of Lion, it will not be long before all of the apps will be for 10.6 and 10.7 only.
Flash 10.1 is actually as fast as 9.0 for video, so I recommend you upgrade. And yes Leo isas fast as Tiger for me, as well with the caveat that the boot time is like 10 sec longer, but that I can live with.
Yeah, it certainly runs great on here... I may even put it on the eMac as well... it has the 1.42 and 2 gigs of ram.
As for Flash, yes....Flash 10 makes a huge difference, especially in online game performance. The games aren't choppy anymore. As far as boot times, I can't tell a difference, they both seem the same but I am going to see.
What's even better is I just bought a dual 1.25 off eBay...it will be here soon.
I talked to a few people and determined that there is a reasonable performance difference between the 1.0 Dual and 1.25 Dual (Probably the 2MB L3 Cache as opposed to the 1MB), but not much of a noticeable difference between the 1.25 dual and 1.42 dual. So hey, for $30, I can't complain! Combined the 1.25 is a 500MHz upgrade, which in the PPC world, that seems to be quite a bit.