What are the main advantages of using FreeBSD instead of Windows?

FreeBSD has no solid power management, especially lacks support for hibernation to disk.
It also does not have much for video conferencing.
Firefox and Opera are both available. IE requires trickery with wine emulator.
Stardict is included, others run or not via wine.

Actually i would propose Linux instead - it has hibernation to disk, fairly complete webcam support like for skype, same browsers as FreeBSD and Flash is not crashing them, it automatically mounts USB flash sticks without enormous effort like in *BSD. 32bit version better than 64bit - but if you have huge disk and RAM >2G you can choose to take this adventure.

You can run some LiveCD of Linux - like Mandriva One, OpenSUSE or Ubuntu to check out how well it does (you can install lots of software in its ramdisk, or run pieces from your hard disk)

What will be initial confusion - there is no C: D: E: etc, all files are in same unified namespace.

While FreeBSD does work great, I believe it's better for servers and linux a better alternative for clients as the hardware support is generally better.

The most important factor in choosing what to use is hardware support.  Check your hardware is fully supported by FreeBSD before deciding to go with it.

I use Gentoo on my Thinkpad and everything runs no hassle.  There were some teething issues at first but all is great now.  Your best bet is to try the Ubuntu livecd as stated above, as I found it is best for support out the box.  You boot up and everything just works !  If you decide to then go with it, you just click the install icon on the desktop.  While Gentoo is my distro of choice and love it, Ubuntu or Kubuntu would be the best choice for someone new to linux IMHO.

Then Linux:
http://distrowatch.com/stats.php?section=popularity

I'd stay away from Fedora - it sometimes breaks stuff etc.
I would choose KDE&RPM distributions - OpenSUSE or Mandriva
I cannot make judgements on Mint - never used it
Ubuntu is in ugly colour

Dreamweaver here (check all your apps same way):
http://appdb.winehq.org/objectManager.php?sClass=version&iId=14343

If you are a complete newbie, I can definitely recommend you ubuntu. It is targeted to people who are not familiar with operating a computer. Personally, I swear by gentoo, since it allows me the most finegrained control. The downside to this is that it takes much more work to set up and maintain.

It is never necessarily to switch distributions. However, if you want to keep that option open I would suggest that you make at least a separate /home partition. This will allow you to reinstall another distribution without needing to backup/lose all your files.

Dualboot is absolutely possible and even very easy. Most distributions automatically detect windows and set up an entry in the bootloader. One warning though, Windows is, as usual, not very cooperative, so you need to set up Windows first, and then install linux, otherwise Windows will overwrite your bootloader.

20gb is a fair amount to get you going for your first time.  I used Linux and 2 other operating sytems on my work laptop and primarily used Linux for day to day stuff and 20gb was sufficient.. even with X Windows installed and MS Office.  You can always use something like GParted to resize the partition at a later stage if the need arises.  

You may want to use 30gb as I use Gentoo which starts as a really minimum install (and customizable support when building, resulting in smaller package installs).  It's not bad to maintain but it can be painful for a person new to Linux to install it.

The above suggestion of mounting of /home on a separate partition is a great idea.  I kept most of my stuff on my Windows partition as Linux reads NTFS great using ntfs-3g.  Simarily, you can install ext3 support for Windows to access your Linux partition.

Virtualisation is another solution.
I would suggest Mandriva as base system and XP as virtual machine in virtualbox which is not exactly included in system (you need one from SUN which has seamless desk integration)

Debian is for punks.
Resize your XP to take full disk and install one virtual machine (virtualbox has best desktop intermixing) with opensuse, one with mandriva one and another with ubuntu desktop LTS, and another with mint 10G disk and 1G RAM is fine for either)
Try to break all of them
Last who stays is your choice for future.
Now you should feel heroic:
Resize XP down using Linux
Install Linux of your choice
Move XP now into virtual machine inside Linux

Hope these excersizes will improve your newbie attitude.

If you have particular interest in the *BSD world, or want to understand how the thing *really* work in Linux / servers, then you should give Slackware a try. This is a linux distribution with the closest BSD system of all. The distribution is purposefully created to give extreme control to the user.
I should warn you that this power doesn't come easy: You will need to learn *a lot* before you can start doing things. But in the process you will get the best knowledge of the basis of the systems that you could get.
Anyway, just my 2 cents...
http://www.slackware.com

If you enter this journey, It would be really good to read this, as it explains the philosophy behind the operating system and gets you started with a lot of things:
http://www.slackware.com/book/