Where can I find themes with fixed width (in px, optimized for 800x600 resolutin)? Design is not important (I`ll change it if I have to).

you can pretty much do this with any theme. all you have to do is go to the theme.html file in the theme folder and define the width on the first table to either 700 or 750.

I did it, and I got full width table (on 1024x768). I don`t know what is the problem...

the theme in question probably has a style declaration that forces tables to 100% width. In /themes/<yourtheme>/style.css look for something like the following:
which says for every table tag set the width to 100% of the container's width.

If you change this to:
your tables should honor your HTML width attribute. Please note that this can modify the output from modules and blocks that expect that their tables will be 100% of the available size. You might have to modify the module's templates for their output to render as expected.