Having some video problems

Its been quite a few weeks since I built my new gaming rig, and everything has been working fine with it, up until today. I had turned on the computer and was launching a game when all of a sudden my monitor went black. I think it must have gone into sleep mode because the orange light on it was still lit (I was using an old Gateway monitor from an older computer we had) and it has pretty much been stuck there ever since. I tried turning the monitor off and back on, unpluging the monitor from the case and plugging it back it, but nothing has worked. At first I thought it was the monitor, so I hooked it back up to the old Gateway computer we have, and it seemed to work fine with that, so I plugged it back into my new rig, but still had the same problem with it. I was fiddling around with the monitor cable and took the graphics card (HIS Radeon 9800pro) out and put it back in and got the screen to work for a little bit, but after the computer booted, I couldnt really see the mouse on the screen, and a few seconds later the picture got all messed up and it told me to reboot the computer, which I did and ended up losing the video again. I'm guessing it is probably the monitor, as it is a pretty old one and the video card was brand new and I havent had any problems with it since I built the computer. Also, I was reading an older post on the forums here and someone had said that their old Gateway monitor's sleep mode function card went bad, so I'm thinking that might be what has happened to mine. I hear the computer boot up ok and play the startup sound, so I know it has to be either the video card or the monitor, and if anyone can help me figure out what the problem is, it would be much appreciated. Thanks!