Since DVD's aren't recorded in 1080i, the only way to view them in 1080i would be using an up-converter. I know that the video cards will display a video in full screen on the monitors, I don't know if they actually will up-convert to 1080i.
For the ATI x1xx cards, catalyst has some settings for theater mode, and I recall seeing some settings about what HD mode I wanted to display, but not having my computer connected to an HD TV, I never played with them. Again, just because it can display an HD image to an HD set doesn't necessarily mean it will up-convert the image for you.
I'm not sure upconverting to 1080i will buy you anything, however, you might see if you can upconvert to 960 (especially 960p) being 2x the DVD resolution.