It is a bug in your motherboard/bios. If you check the output of dmesg and look for the bios e820 map, you can see exactly what memory the bios is reporting your system has. Typically the memory addresses between 3.2 GB and 4.0 GB is reserved for addressing hardware, such as video memory. A proper motherboard will move that ram to higher addresses above 4.0 GB so it can still be used ( by a 64 bit or pae enabled kernel ). Some don't, leaving that chunk of ram unusable.
The error indicates that Ubuntu does not provide a package named "linux-image-amd64".
I surmise this is due to the fact that the guide from where you picked the shell commands has been written for a Debian installation. Indeed, if you search on debian.org for this package, you'll get a positive result. This is not the case if you enter this package name on http://packages.ubuntu.com/
The last command should be
sudo apt-get install linux-image-generic:amd64
or (if you really care about latency)
sudo apt-get install linux-image-lowlatency:amd64
Please note that the 32-bit kernel will be removed. Also, some 64-bit userspace libraries will be installed. To see what apt-get will do to your system without actually touching your system, run apt-get with these options
sudo apt-get -Vs install linux-image-generic:amd64
If you want to retain your 32-bit kernel as a fallback, install a different kernel image, e.g. for your 14.04 (codename: trusty) try:
sudo apt-get install linux-generic-lts-trusty:amd64
sudo apt-get install linux-generic-lts-utopic:amd64
Best Answer
Ubuntu will automatically turn on Physical Address Extension (PAE) if it is available; otherwise it will not even install.
Wikipedia, important parts (see the link for more):