http://www.geeks.com/details.asp?invtid=KS-A022&cat=TOO (Alsot at Amazon through geeks.com)
"Xbox and Gamecube have VGA boxes that can be used with monitors, but the problem is that only progressive scan games will display using them. You’ll have the same problem using that adapter that you linked, so even if you got this working you can’t play every game unless you have a deinterlacer in there… same goes for some dreamcast games.
The best solution I can think of is 2 converters (RCA-to-VGA) and (component/console to VGA) and you switch them based on which game you’re playing…"
The XBOX 360 has a cleaner solution. No limitations are mentioned for the official XBOX 360 VGA Kit