Here's what I'd do to mount stuff:
First open a console and run
Code:
$ fdisk -l
You'll get a bunch of tables with device (partition) names like /dev/sda1, /dev/hdb1, etc. Have a look at the filesystem and size of each one to determine which ones are partitions you want to mount. You need to make sure that mount points for each partition you want to mount exist, so run
Code:
$ mkdir /mnt/{slave1,slave2}
substituting slave1,slave2 for directory names pertinent to the actual partitions.
You need to be root to mount, so run
Code:
$ su -
$ mount /dev/sda1 /mnt/slave1
substituting /dev/sda1 and /mnt/slave1 for appropriate device and directory names, and you're set.

As for where to find the console in Gnome, it's in one of the top panel menus. In Breezy Badger it was under System > Administration, but I think it moved to the Applications menu in later Gnome releases.

nVidia's Twinview is very good for dual monitors. I had Compiz set up on dual monitors and rotating the desktop cube looked pretty cool, it was one big 'cube' across two displays. ATI has had Xinerama support since 8.19.10, but ATI's drivers and the reverse engineered open source ones have always been a little dodgy by all accounts.

When it comes to desktop environments, it pays to learn the various terms:
  • Window manager: controls how windows behave, things like grabbing focus, minimization/maximization, inter-desktop movement, etc.
  • Window decorator: controls the appearance of the borders, titlebars and buttons of the window.
  • Toolkit: Controls the "look and feel" of the display environment. Provides a framework for the things like messageboxes and window buttons/menus. You'll probably only have to deal with Qt (which KDE uses) and GTK (which Gnome uses) right now, but there exist a few others, like E17's. This is the main differentiation between the appearances of different environments.
  • Desktop environment: The whole package. Comes with everything needed to display stuff, and usually a whole lot more that you don't need.

I hate desktop environments because of all the bloat they come with, but I couldn't run Compiz-Fusion with any other window manager so I went for the lightest DE - Xfce, which is quite nice. At the moment I'm using E17, which is nice. I Love Fluxbox, but that's geared towards a more experienced user so you might not want to tackle that just yet.
Here are some screenshots:
<a href="http://enlightenment.sourceforge.net/Enlightenment/Screenshots/DR17_Screenshots/_images/e17-28.jpg">E17</a>
<a href="http://www.osnews.com/img/1347/myKDE3.jpg">KDE</a>
<a href="http://commandline.org.uk/images/posts/gnome/gnome-screenshot-large.png">Gnome</a>
<a href="http://img.osnews.com/img/4091/xfce4.png">Xfce4</a>
<a href="http://www.srijith.net/trinetre/archives/upload/screenshot-fluxbox.png">Fluxbox</a>