I think you get the same outcome by clicking on the channel you want in the channels palette and then going straight to Image>mode>grayscale - it will automatically ask if you want to drop other channels and convert to grayscale in one go.
Image>adjustments>channel mixer allows you to mix the various channels to get the perfect balance. Just check the monochrome box at the bottom of the channel mixer pop up and dial in what you want - say 50% red, 50% green. As a general rule just make sure it all adds up to 100% to maintain the same brightness.
I have read that for a great Ansel Adams effect in a mountain landscape you should go with Red = +160 Green = +140 Blue = -200 (still adds up to 100%).
Channel mixer can also be an adjustment layer which means you can make changes whenever you want and use masks.
I did this conversion using one mix for the building to maximise contrast, and a different mix for the sky to minimise noise. You'll generally find a lot of the noise is in one channel.
http://www.deviantart.com/view/13561565/
You can experiement with the channel mixer forever - it is the most flexible way I know to do B&W conversions.
Edit : because I worked out my image linking issues!