or can I has more invites?
I think that is a pretty great system for earning invites, just make sure the number is pretty high.
I'm not sure it is. Karma may be just an ego thing on the one hand, but on the other it tells others how much they can trust you or appreciate your contributions. I think treating karma as currency is a mistake.
Invites being tied to karma is a good idea, but without modifying the number.