I am a 32bit user, having gone from XP, to dual-booting XP and Ubuntu and finally using only Ubuntu.
Depending on what you use your computer for, you may want to consider dual-booting. If you use a lot of Windows applications, there are programs that may let you run them on a Linux distribution. Wine, Cedega and CrossOver are applications you may want to read up on before making any sort of switch. You may also want to take a look into virtualization software, such as VMware. I have no experience with virtualization though, so I can't say much on the issue.
Personally, I use Wine for WoW. WoW works fine aside from the cursor being slow, which is due to Blizzard not (in a nutshell) allowing Hardware Cursor to work in OpenGL mode on the Windows version of WoW. The game has to draw the cursor, and since my rig is older, my cursor moves slower in areas where I get a low fps (outdoors, usually.) The kicker (from what I've read) is that if you're running on a Mac, OpenGL does allow for Hardware Cursor. You can run WoW (in Wine) as Direct3D (d3d) - this allows for Hardware Cursor and sometimes a higher fps, however most people have texture or other issues. I installed WoW via instructions here: https://help.ubuntu.com/community/WorldofWarcraft
The one thing that kept XP on my computer was my printer and scanner. You may want to look into making sure that your printer is compatible with Linux - http://www.linuxfoundation.org/en/OpenPrinting is a good resource however I think the reports on compatibility are user-defined. My printer was said to be a "paperweight" according to that site, however some forum searching proved that it would work upon installing a particular driver.
As I'm not a 64bit user, I hope that someone here with 64bit and Linux knowledge could answer your question. Alternatively, you could sign up on a forum like ubuntuforums.org - they sometimes can provide information on other distributions and have a very large forum userbase.
If Ubuntu runs 64bit (which I'm sure it does) I'd suggest Ubuntu. Then again, I am biased as it's fulfilled my computer needs and any problems that have arisen have been solved through the help of a great community.
Last I recall Blizz was delving into LinuxMint. Blizz may be able to provide insight on that distribution. From what I've read, it is different from Ubuntu mainly in the fact that it's a bit snappier in appearance by default and it comes with a particular codec package automatically installed. These same codec package is indeed found in Ubuntu, but are left uninstalled. The codec package can be installed very quickly so that should not be a make-or-break factor.
For Linux64, only one real question. Do you browse or frequent sites that use flash? Currently there are a few hackish ways to get flash to work in 64bit Linux, but from my experiences on my laptop, even when setup correctly, it's still buggy and occasionally still won't work. But if you avoid flash sites like the plague, then FF works nicely in 64. Now as soon as either flash has a proper 64bit release or when one of the flash clones was a stable 64bit version, then I'll reload Linux64 on it again.
Yes, you can. But take backups to be safe. Make a partition for Linux, depending on which distro you pick, most installers will offer you a chance to set up a bootloader and flag which OS you want as default etc.
Just so you know, should this "break" your windows boot up, which has happened to me (hard drive - motherboard issues), you can pop the xp install cd in, drop into the repair console thing, type fixmbr I think it was (should you need it, I can find the note). (This is of course when you drop the bootloader info on the mbr)
Wubi was my very first taste of linux. It doesn't create a partition on your drive, it lives in a folder in your windows installation. You can still dual boot with it, it's just not quite so scary to install it. I fell in love with it and have been with linux ever since, 47 distros later, I chose Ubuntu.
Been messing with some themes. My dock isn't done yet, can't even decide if I want it so i won't show it, but here's one picture.
What programs are you using to style your desktops?
Emerald is a theme manager - similar to WindowBlinds but free. CompizFusion is used for desktop effects like the way windows open and close. Icon packages are handled defaulty, as are cursor themes if I'm not mistaken.
Blizz - I love those icons, but I haven't been able to find them anywhere. :( Would you have a direct link? I've browsed GnomeLook.org but haven't found anything.