Thank you about your helpful definitions and descriptions about dpi... now I would like to hear from the person who posted the orginal questions... about using an old computer with an old graphics card that will not output an adequate resolution. I think I know about dpi after 31 years in the business, but I am not sure the person making the query does.
He apparently believes that changing the dpi ("...mount up your dpi in your screen...") will enable him to reach the resolution he needs for gaming... when what he wants is apparently impossible for that Compaq Armada 1750 (Value Line Pentium II, 333 MHz with 66 MHz FSB, and 24 bit 1024 x 768 when the 4 MB of installed video memory is utilized.)
Sometimes it is not helpful to keep searching for an answer that is satisfactory, but DPI isn't going to bypass the limits on that 12.1 inch TFT Active Matrix screen with 4 MB of video memory.