The X2VGA has composite video and digital audio outputs, for if you wanted to connect your xbox to a TV also. (This also comes in handy if you use a Video Capture device of some sort and want to still be able to connect it to your xbox wiith your xbox connected to your monitor.
The X2VGA+ has an audio and vga input, along with an audio and vga output.
This is so you can connect your pc and audio from your pc to the X2VGA+ and then connect the X2VGA+ to your monitor and sound setup. You can then by the press of a button on the X2VGA+, switch between your Xbox and your PC using your sound and monitor.
If you want more details, just look at their manuals.X2VGA ManualX2VGA+ Manual
So if you don't have a video card, or have no reason to need composite video output with your xbox already connected to your montior. Then buy the X2VGA+.
Have you found a place to purchase the original X2VGA? The only place I have found is EBay on occasion, but the bids get up to $60-70.