It may not be 20% of the internet, depending on who actually uses your site/app. Check your analytics.
IE6: 5.7% IE7: 11% IE8: 1%
FX1: <1% FX2: 3% FX3: 63%
SF: 2% OP: 5% CH: 6%
Really cool how Chrome has overtaken Safari, Opera, and IE6 (Admittedly it's a webapp, for which Chrome is well suited).Also Firefox does a really great job of upgrading its users to the latest version.
At the moment I support IE6, but I'll probably stop soon. It's just too much extra work for not a lot of payoff.