blue is definately vga although you get black aswell lol. Dvi is digital vga is analogue so dvi has better picture quality through less data loss in theory.
you can hook up two at once with vga all you need is two ports on the graphics card. Digital is numbers so is less likely to be corrupted as its 10101010101011000111 and has a sum at the end to check the whole stream so that it can be easily repaired. analogue is a wave picture a sound wave any degradation in the signal cannot be repaired. DVI is better then vga so just concentrate on that in simple terms
You need a graphics card with two video ports (either two dvi, two vga, or one dvi and one vga). You could get a vga or dvi split cord (forgot the technical name for it) and plug it into one video port but the two screens you hook it up to will display the exact same picture. Ppl usually only use it to display power points on a projector while watching their computer screen.
If you want to dual monitor then you need a graphics card with two video ports. Just make sure you know which ports you need (check what you have on your monitor).
example: This graphics card has two dvi ports. So you could say get two of these monitors and plug each one into a different dvi port on the graphics card.
Then you just install nvidia control panel (the software that comes with the graphics card) and enable dual monitors. If you have the screens where they are on the wrong side and you want to switch them, then you right click desktop -> properties -> settings and drag one of the monitors to the other side of the other monitor and you can change the primary monitor (the one with the task bar at the bottom) through this window as well.
With dual monitors, you get more desktop space. You will be able to mouse between the two monitors simply by dragging your mouse between each monitor.
Its great when you're maybe watching a movie and talking to someone on aim at the same time. Or you could be typing up a report or something in word and surfing the internet on the other monitor.