You will need a dual VGA splitter box like this, since a regular VGA splitter will not work. http://www.google.com/products/catal...d=0CDcQ8wIwAg#

You can use any cap card with the following set up.

XBOX-VGA cable from XBOX to SPLITTER. VGA to X (where X is whatever input your cap card uses) from SPLITTER to CAP CARD. regular VGA from SPLITTER to MONITOR.

This is a basic setup that is pretty inexpensive. I have used it before and it works. This also assumes that you have no output on your monitor. If you do, it may be easier to output from the monitor to the cap card. It may prevent screen lag, though I don't really remember if the splitter causes any in the first place.