I have picture but it is very bad, on 1920x1080p i can barely read letters on the internet =/ on 720p it is better but still the picture quality is low...
It is not clear what you mean by this. Many people find HDTVs - any HDTV - poor displays for work/text/browsing and so on - even when the picture is perfect. The size of the screen and the distance you sit from it can make reading text difficult for lots of people.
Unlike HDTVs, which are really designed for continuous motion video, computer displays have much higher dot densities, and excel at static displays, which is one reason why a 30" computer LCD screen costs *5x more* than a 32" HDTV LCD screen.
You are also trying to feed your PDP-4270 resolutions with which it is "compatible", rather that its native resolution, which is 1024x768. Try running at the *native* resolution of 1024x768 and see if things are better.