Information overload, but I think I am starting to understand

I am a little confused about the hard drives though and what is compatible. You say IDE to USB, is that for connecting an external USB drive? Could I buy any IDE hard disk if I wanted?

Then with these WHDLoad games, some of them will work with kickstart 2.05, but won't some also be only for kickstart 1.3? I got a degrader floppy with my amiga 600, so would I still be able to use this even if I have this HD setup you recommend?

Finally, the RAM upgrade, the only one I can find is this one: A602 but I am not sure this is a) correct and b) where is this installed (looks too big to fit).

TIA