1) They'll be referring to virtual machines. What this means is that you run some software (like VirtualBox) that simulates a completely independent computer running in a window. So yes, you can run Linux in a virtual machine on Windows. You can also run Windows in a window on Linux. There are a few things to be aware of here. Firstly, since Linux is free, you can install as many virtual Linux machines as you want...but since the virtual machines function as if they were real computers, the same is not true for Windows: you will have to register and "activate" modern versions of Windows for the hardware onto which you've installed them...even if it's virtual hardware. The problem here is that your virtual machines are *not* identical copies of your real machine. The software uses its own simpler virtual hardware setup as an interface between the installed OS and your real machine. So, if you think you can simply wipe Windows off your physical machine and then run it in a VM, you may be mistaken: some "OEM" licenses are specific to *one* machine...Microsoft will not allow you to remove it from that machine and run it on another...not even a virtual machine. Windows will detect it's on a different computer and will fail to activate. The other implication of this "virtual hardware" limitation is that some applications perform much worse on VM's than on real computers. Word processors and so on typically work exactly the same in a VM as on a real computer: if you expand the window to fill the whole screen, you can't even tell you're using a virtual machine. 3D games and so on, however, can be a completely different story. "Do the benefits add together?". Probably not: you're more likely to work efficiently if you're building *one* set of habits (shortcut keys and so on) to work with *one* set of data, than mucking around between two different systems. However, while you're learning Linux and still depending on Windows, it can be good to do things this way. For sure, try Linux in a virtual machine first, if you can: just remember it will likely be faster and nicer as a physical install.
2) Yes, so long as you don't wipe out your Windows installation! If you do a physical install of Linux, make sure you put it on its own partition so you can "dual boot" with Windows (choose, from a menu, whether to run Windows or Linux when you start your computer). Popular Linux distributions give you the option of setting things up this way when you install them.
3) http://appdb.winehq.org/ is the WINE application database: it's a fairly comprehensive list of popular Windows programs (especially games) with information on whether and how well they work on Linux. Bear in mind that it can take the WINE developers a while to get things working with new titles. So, occasionally a brand new Windows game will "just work" under WINE on Linux: sometimes they even play at the same speed. Often though, there'll be some problems...and sometimes the game won't run at all, or even install. However, if you wait a few months, you'll often find that people have managed to get the game working, so even if your favorite recent game doesn't currently run on Linux, it may next year.
4) A few hardware manufacturers ship systems with Linux desktops pre-installed, but the most economical way to buy a Linux machine is to buy the machine with *no* OS and simply install Linux yourself. Friendly modern distributions like Ubuntu install from DVD's just like Windows...in fact, they're even easier, in the sense that the Linux distributions tend to have all the necessary drivers for (reasonably common) hardware on the disc, whereas Windows installs tend to involve lots of downloading or grabbing discs for other drivers.
5) Linux is basically immune to Windows viruses. Generally speaking you just don't get viruses on Linux. They could become an issue at some point if Linux becomes much more popular, but Linux is a secure and rare OS so it's just not a good target for virus authors: many Linux (desktop) users just run their systems day after day for years and don't even think about viruses. If you run infected Windows software on a VM or in WINE, you could infect *that* part of your setup...but that's not your real system. WINE can be trashed and reinstalled without touching the rest of your Linux system, and (since any virus you run in WINE thinks it's actually running on a Windows machine) even if a Windows virus had write permission to mess with your Linux system, it wouldn't know what to mess with.
Should you switch to Linux? Yes, if you have plenty of time to learn a new operating system. It will take time and you will sometimes be frustrated when the things you expect to work as they did in Windows just don't work for you. However, if you persevere to the point where you're comfortable with Linux, you'll never go back to Windows. Why on earth would you *pay* for software that makes you do things a particular way whether you like it or not, when for *free* you can use software that does whatever you tell it to do? For geeks it's a no-brainer, but if you don't have a lot of time or interest in technology, it can be better to stick with Windows.