Originally Posted by Suikaz
[...] What exactly are the system requirements/recommendations besides that it must be 10.6.X or higher?
[...] How much RAM do we need? How much free space do we need? Will the game run smoothly with 1GB of RAM, or will it lag incredibly? [...]
The Windows requirements suggest 2GB of memory for Vista or 7 users. While Snow Leopard is pretty lean, I think 1GB for gaming might be a bit constrained. My 2007 MacBook Pro had 2GB of memory and Diablo III was "almost" playable (although that was probably more due to my GPU, which I'll get to in a bit).
My installation of the PBE is taking up 3GB, but keep in mind it'll need some extra room for unpacking files for installation (which is then freed up afterwards). I'd aim for having at least 10GB free at all times, just because you don't want to end up in a situation where there's no room for temporarily files (not just for LoL, but for all applications).
The biggest thing, especially with older Macs, is going to be which graphics processor (GPU) that you have. Any DirectX 9 capable GPU should be able to "run" the game, but not necessarily very well. The Windows recommendation is a Geforce 8800 with 512MB of dedicated video memory (VRAM). Most MacBook Pros from before 2011 are going to have 256MB of VRAM. You also want to avoid Intel graphics, unless you have a Core i5 or i7 with HD 3000 or 4000 graphics. (Note: NOT the X3100 graphics!) As an example, my 2007 MacBook Pro had a Geforce 8600M with 256MB of VRAM, and Diablo III was only "almost" playable - at all the lowest detail settings.
You can determine which GPU you have by clicking the Apple menu and selecting "About this Mac", then "More Info" from the window that comes up. If there isn't a section for Graphics on the new Window that's open then you'll also need to click "System Report", and under the Hardware tab select "Graphics/Displays".