Promotional Contest (need ideas)

I was hoping others who own a hosting company could help with ideas for this.

I have several Dell Lattitude laptops, they are not brand new but in great condition. They would not be much for running games but fine for general business purposes.

I thought of running a promotional campaign where the person who brought in the most new accounts would win one of these laptops.

My question is what would be fair? Run it for a month, but I thought there should be a minimum of (5?) accounts to qualify?

Anyway, any help or ideas would be appreciated.