Yup, i have my comp hooked up to my tv through a 7800gt. Just run something like a TV utility, which im sure your card has. It should detect the TV, and automatically run it at the highest resolution possible.
720p and 1080i(p) are the two highest resolutions MOST HDTV's support. Although screens smaller then 42" tend tonot support 1080i or p.
Monitors can run quite a bit higher then that. Infact most likely the monitor you're using right now is running higher then that.
I should point out something that I found funny-
"I AM NOT talking about the resolution in the display tab in the control panel where you can choose 600x800, exc. I am talking about how can I change it from like 780i to 780p or to 1080i ,exc."
That sentence makes zero sense. Because you ARE talking about the resolution hahaha. What did you think 780 and 1080 stood for?