[thejeff]

I think the claim that CO2 is more effective weight for weight was against water vapor not methane.

According to Wikipedia (Not an authoritative source, I know, but usually reliable on basic stuff), methane has 25 times the impact of the same amount of CO2 over 100 years. This appears to be a common way of measuring the effects of emissions. It is much stronger initially, which is probably where your 50x comes from, but oxidizes in the atmosphere into CO2 and Water with a half life of 7 years