odds on you'll be rewarding a bot, why not give everyone a ticket per post that they have made and have a draw for a prize. The contributors have a benefit based on their contribution and those that are bots with 1-2 posts before ban get little chance of winning.
I'd much rather give out a Tom's Hardware hammer or something nifty as a prize for a contribution, than as a tertiary by chance door prize. Right now we're running the BestConfigs over in the Systems area, and the top folks in each category will probably get a nifty hammer.
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