Different ISO modes

What exactly are the different ways of running isos? What is the difference in the M33 driver and Normal mode and the Sony NP6640 or whatever it is?
How come some games work with some configurations and others work with different ones?

I am not sure. I haven't yet encountered a problem yet, unless some games specifically need you to update to the newest FW. I tend to keep my M33 setting to Sony NP6640, seems to work the best. A good guide about M33 settings can be found here: http://www.psp-hacks.com/forums/viewtopic.php?id=119276 (I wish there was a guide on pspbrew, so I wouldn't have to keep linking that site--if you know what I mean?)