If Xamarin programing is going to be how you make your living, then I highly recommend this list of hardware and software.
iOS, OSX, Android, Windows Phone, Windows
Ideally, you’ll run Windows on a Mac, using Parallels or something similar. Parallels is the right thing, but you need hardware that will fully support dual operating systems.
The solution is to get serious and buy a MacPro (desktop) with enough memory (32GB) and enough Cores (6) to never experience a slow down. You’ll also want to get a lot of storage, I recommend at least a Terrabyte of Flash storage. This configuration will set you back just over $5,000.