1: A 2GB SD card is recommended. Not bigger as that's SDHC and not supported by all homebrew.
I would recommend having one for the Wii only as some apps use it to save stuff to.
2: If you have an older Wii, you should use the search to find the guide to ripping and burning discs. If your wii is rather new it won't be able to read burned discs in any way.
I still recommend USB loading though, even on older Wiis.
3: Most programs are made for PC, I guess you can run them with bootcamp or find an equivalent program for the mac. But I've no real experience with macs so I'll let someone else answer this in more detail :P