Ok I just installed the Homebrew Channel and Bootmii and made a backup of nand and keys, I tried running my US GC discs and they weren't readable so I take it that I still have stuff to install? something like Gecko OS? (is there a best choice?)
Edit: Just did some searching around, StartPatch seems nice because it runs from the disc channel, so should I install IOS236 and StartPatch (or are there better choices?)
I would use priiloader since that gives you a brick protection too. You can also use gecko os or neogamma to play other region gamecube games, they are some nice neogamma autoboot channels which automatically boot the disc in drive like the disc channel.
Most should be fine once set up correctly. What's troublesome are the specific Japanese GC games with errors running under incorrect language mode. I've got a chipped GC to know about this.
For affected titles you'll want to find some way to trick Wii into thinking it's a Japan system. I'm not sure of the specifics how to do that though...
Radirgy as an example. When in English setting, it cannot save progress. The game waits upon attempting to access memory cards. You have to remove all cards then it'll resume normal play.