I have just installed Ubuntu onto my computer. When i installed it, my screen resolution was at 1280x1024. Now that i've rebooted it, it defaults to 640x480, and it won't let me change it back ... or to any other resolution for that matter.

How do i add these resolutions so i can use them?

Please post the contents of /etc/X11/xorg.conf
What you are looking for is entries like this:

SubSection "Display"
Depth 1
Modes "1600x1200" "1280x1024" "1280x960" "1152x864" "1024x768" "832x624" "800x600" "720x400" "640x480"
EndSubSection

What this is showing is all the resolutions available to you--depending on how the installer detected you monitor, they may or may not get set automatically.

Alright ... I have pretty much the exact same as what was written above, except my monitor can't handle 1600x1200. I have:

SubSection "Display"
Depth 1
Modes "1280x1024" "1024x768" "832x624" "800x600" "720x400" "640x480"
EndSubSection

I also have 5 extra SubSections with a Depth of: 4, 8, 15, 16, and 24 .... all of which have the same Modes.

Above the Subsection "Display" i have a Section "Screen". All of the contents of the Identifier and the Monitor are correct (as in, it recognizes the right video card and the right monitor.

I'm assuming that the different modes are the resolutions that my particular monitor can handle, yet it still won't let me adjust it.

Hmmmm...What do you see when you open System-->Preferences-->Screen Resolution?