Pow will alter your DNS resolution to have all domains that end in .dev or .test be routed to the local machine (to hit pow).
If you want to use MAMP as well as POW you'll need to turn off pow temporarily for it to be useable (Or you could change the MAMP settings use another port besides 80).
To turn off pow i'd recommend installing the powder
gem:
gem install powder
powder down
You could also use pow to serve up static sites, you'll just need a blank config.ru and a public folder to get going. More info in the pow manual here: http://pow.cx/manual.html#section_2.4
rvm list known
is the list of things that RVM can install. – Navar