Sorry if this is a stupid question... then how does Leopard make use of 4GB of RAM on this Macbook?
Is addressable memory space the only real benefit?
Though Leopard is a 32bit kernel, the OS was reworked/tweaked in order to allow 64bit programs to run, and allow for greater addressing space. SL is to be full 64bit BTW, so the kernel will become 64bit as well.
The increased addressing space, also means you can run larger files. Not just more memory.
No, it won't speed up everything, but here's a simple way to look at it. You can process 2x the information per clock cycle.
So some programs can benefit, such as math intensive applications (rendering, simulation, engineering, scientific,...). This also includes things like audio and video playback.
Even games, if you indulge, so it's not just for scientists.
Hope this helps.