Linux users don't like to pay money for software (I know i wouldn't like to pay for anything on my linux box), so you'll have quite a tough time coercing them to pay up.
Err. No.
I'd say it's about the same for most OS's. There are probably about the same percentage of people who would pay for a game on linux as windows.

A lot of linux users do "pay" for their software. They just don't do it in money most of the time. They do it by putting back into the open source community. {Barter is alive and well }

I know lots of people who would pay for a good game on linux.
Particularly if you gave them like a shareware version first.

Armand