how do i do this??? do i still have to buy some cable or what??

Ive seen a couple of cables/hardware on here that should do the job.. http://www.gamealliance.co.uk cya..

I do it "The Thug way". A simple cable-run through my video capture card (peice of crap ATI). I don't have a VGA box so the quality isn't that great but it works..

You might want to check www.liksang.com they still have many
new dreamcast accesories and games, you will definitely love the improvement you'll get in visual quality from running your DC through a PC monitor.