Help with running an ATI and nVidia card together

I've been using a GeForce 9800GT as my video card but I just bought a Radeon HD 3450 so I could use my 42" HDTV to watch movies on. Now, I know its not possible to run them together on Vista, but I'm on XP Pro x64 and I've heard that its possible. I have gotten the system to run with both cards in at the same time, and although if I plug my monitor into the 3450, I can get a display with the nVidia(primary slot) and I can extend it over to my TV. The nVidia control panel doesn't recognize the TV, but extending it through windows will bring it up. However, as soon as I drag any video over to the screen I get a black box where the picture should be and then a blue screen. Sometimes it catches the last 1/2 second of audio and loops it until shut down. Thanks in advance for any help, as I seem to be at the end of my rope.

The 9800 doesn't have HDMI out. Thats why I got the ATI card. For now I've just been running the ATI card without the nVidia in at all. I suppose I'll just have to wait until I can afford a nicer new card with HDMI out.