I have actually made this exact transition (from Windows C++/C#/PHP/Perl) to Ubuntu/Rails.
I am not too sure about remuneration, because it depends on your geographic location. In Silicon Valley Rails is very popular, and developers get paid well. I suppose some areas have more demand for .NET(C#) or LAMP(w/PHP). It also depends if you want to work remotely.
It seems to me you might want to think about the future: the world is changing fast, and there are millions of people learning programming who will compete with you. Which technology has a brighter future? Which is likely to be in high demand with low supply of good coders? How can you create value for your employer that will not be outsourced to someone who works for 1/4 of your salary?
My experience is that Linux is not very popular with good Rails developers. If you watch http://railscasts.com, you'll see that they use Macs. I went to a few Rails meetups, and everyone seems to be working on Macs. Ubuntu is sort of the poor man's Rails Dev machine. It works fine, but costs 1/4th as much as a Mac.
By the way, I find editing the files on my PC (for now!) using Notepad++ and sharing the files from the Ubuntu machine works very well. You get all the benefits of working on your normal Dev machine and all the benefits of working in Ubuntu. I tried an IDE on Ubuntu, but it was slower than hell and didn't work for me. I love Visual Studio, but it's not really necessary if you do TDD.
I would suggest doing some freelance Rails projects. Working remotely is good because it is not obvious that you aren't really sure about what to do. Then you can gain real world experience and get better faster. And do all your development TDD, because you will improve and it will become a good habit.
Don't expect it to be quick and easy. I found Rails and particularly Unit::Test to be very difficult to learn. Much more difficult that C++, for example.
As far as areas to get up to speed with, TDD is the only one I can think of. You may want to learn the popular TDD tools, like RSpec, Cucumber and I think there is one very popular tool for stubs/mocks. (That would be a great Stack Overflow question.)
It also depends on your personality. Do you want to learn new (and exciting) tools and technologies? Or do you want to stick with what you know and become an expert? I'm not saying one is better than the other. But if you hate learning new things you would probably not be very happy as a Rails developer because it is changing so fast.
I don't think there are any real clear answers to this question. But it is a great question to ask (and most people don't even think of it -- +1 for Programmers.StackExchange). Hopefully this will help clarify your ideas and lead towards a solution.