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The best place to go for SNR numbers is the manufacturer's specifications, but not all manufacturers publish their SNR requirements. In general, I find that 20 to 25 dB for maximum data rates and as little as 3-6 dB for minimum data rates are common.

I would be careful when measuring SNR using NetStumbler. I am a little skeptical of its accuracy. I've seen with some cards that it always measures a constant noise level, which can't realistically be the case.