Is this possible? I want to use my PlayStation/N64 on my HD monitor. I'm not sure where to look for a cable that converts those circular aerial plugs into something that will connect to the screen though.
Depends on what you're monitor supports you're best chance is if your monitor has a composite/svideo/scart input. Anything else is going to require some special converter (which is likely to cost a fair amount).
If your monitor doesn't support composite/scart/svideo you might be better off getting a cheap tv (freecycle!)
You're not going to get much better video on your HD monitor than you would get from an older TV.