Ok, basically you need something called a VGA Box. Prices vary.
When you have one of these VGA boxes, you have a switch which can switch your TV monitor into the gaming one, and vice versa. I honestly do recommend use a VGA box over a Tv card, only because you get true resolution, and the connections on TV cards are fiddly.
If you can, can you please tell me how. I don't have a n HDTV so I want to use my PC monitor to get high-definition graphics.
Does your monitor have a DVI input on it? If yes, just buy a HDMI/DVI cable and your good to go.
Wouldn't the monitor need to be HDCP compatible? Would that only matter if you were watching a movie? In any case.. you can get a cable cheap from monoprice and give it a try. The cable will come in handy down the road if it doesn't work correctly. If your monitor is only VGA, Here is another option for what Raz mentioned to go from component to VGA: http://www.vdigi.com/... But yeah, you will still need to buy the component cable on top of that price. Yuck!
If your monitor supports it, just get a HDMI/DVI cable.
Here's how I used to have it set up before I got an HDTV: I hook up the PS3 to the monitor with the HDMI/DVI cable. Then I needed sound because I monitor didn't have built-in speakers. I used the old analog (red, white and yellow) cable, plugged it into my PS3, then used an audio coupler (it's like an adapter that binds the red and yellow [for audio] together into one jack) and plugged that into my PC speakers.