I would most certainly recommend a well established PAAS provider. Like maybe Heroku or AppFog. They will make sure you only have to worry about developing your next cool app rather than being a sysadmin.
Heroku also supports deployment by git which is nice. Another big advantage is If your application actually kicks off and you need to scale, scaling with these PAAS providers is often as simple as playing with a few sliders!
1 . There are a lot of noSQL databases available MongoDB and CouchDB are pretty good. They have good documentation. But make sure they fit your exact need before choosing either. RoR is again a great framework. Notably used by twitter. And of course leave the actual server administration to PAAS providers!
2. Checkout Heroku or AppFog. Both of them support RoR and are well reputable with good documentation and community available.
3. I would recommend using Node.js if you plan to build an app which handles a lot of concurrent connections. Like a chat app for example this is basically where Node.js shines.
And finally, Don't think too much into what the hip technologies are. Choose what you are familiar and comfortable with. And most importantly choose what fits your exact need.