Now buckle your seat belt this is going to be long.
When you run a program like Debian from a virtual box, your computer is still ALL windows. virtual box makes a fake computer that you can install things on inside your windows environment, this is very easy and very low risk. in full screen mode it gives you most of the real Linux experience and adds only a few headaches.
When you rearrange your boot order and reboot into Linux. You could be changing things. if you install you are dividing your hard drive in to real parts and having a half (ish) Linux computer and a half Windows computer. This can be a good thing! It will give you the whole complete Linux experience. The big caveat is you are re-arranging your hard drive. Debian does a pretty good job of helping you not make a mess, but if you muck it up you can potentially delete your hard drive and Windows.
It really depends here how comfortable you are with trying new things. If you computer is backed up and you follow the directions it is the easiest thing you have ever done and it should work out fine. there are all kinds of tutorials on youtube about how to install. I even made a website that has only 15 total steps to help my mom install linux. http://mom.super-nathan.com
Although that is Lubuntu not Debian. (very very similar to what the RPI will look and feel like though, you might think about trying it instead of Debian.....)
As far as your architecture let me break it down for you. Debian is ported to a LOT of non mainstream stuff, some of which I dont even know what it is.
Arm-thats what the RPI uses.
AMD64-Thats new computers. any later core duo and ALL i3 i5 i7 computers
I686 or x86 - these are good old fashioned home computers
(x86 Linux WILL work on an AMD64 processor fine, the reverse is not true, download this)
powerpc-is old school g4 g3 macs
The rest I dont even worry about.
As far as a Desktop Environment or a Window Manager, (almost the same thing, its not really important right now)
These are things like LXDE, KDE, GNOME, Blackbox, Openbox (my favorite), FWM, TWM, Awesome, enlightenment, and others. These are only on linux. During the debian isntall there is a choice where you get to install a "graphical window environment" I think they call it. That will be one of these, but it WILL NOT BE what comes with the RasPi. Thats ok you can add it later, its easy!!!