I connected my 40"D300A sony lcd tv to my pc by hdmi port with an hdmi cable(OMEGA brand made in china).i can use my pc and quality is same as vga port.but when i want use hdmi setup in tv menu.tv can not detect any hd device .

Unless you are sending a resolution such as 720p or 1080i that your TV will accept over HDMI it will not see it. Check your user's manual to see what resolutions it will accept.
It is very possible that you will not get any better quality over HDMI and if running PC applicastions the edges of the screen may not be visiable due to overscan.
AFAIK your TV is a full 1080p model so using 1920x1080 over VGA with your PC desktop also set to 1920x1080 may provide the best quality especially if not using HD resolution video as a source.