Your iBook is about the same as my Mac Mini. A bit faster, actually. For me, Leopard is a little slower than Tiger was, apparently because it needs more RAM. I notice more delays when switching apps than I did in Tiger, for example, which is a telltale sign of virtual memory swapping. It's not a deal-breaker, though. It's a minor inconvenience at its worst. You'd probably benefit from more RAM (I think your iBook can support up to 1.5GB), but you don't need more RAM.
It seems to me like the graphical changes make no difference performance-wise. The fancy Core Image effects, like the blurry-translucent menu bar, are disabled on old hardware, so that's a non-issue. There's nothing really fancy about Spaces, so it works just fine. Time Machine's whiz-bang interface is slow, but that's not a big deal since you don't need to use it at all to take advantage of Time Machine — I prefer to just navigate the backup folders in the Finder, personally.
The performance hit from Tiger to Leopard seems smaller than it was from Panther to Tiger. Overall, I'm pleased with the upgrade.
