I have a Samsung LE-37R87 with a native resolution of 1360x768. I have a 9800GTX when I connect via VGA the image is SO much sharper. DVI has vibrant colours but is not sharp at all. I had to mess around in the NVidia control panel for ages to creat a custom setting and even then I had to pull the image out to fit the side of the screen. It SHOULD have fit at 1360x768!! Whats going on? I should look better shouldn't it?
Your set likely is doing 1:1 pixel mapping while using VGA input. 1:1 is essential for seeing clear fonts, etc. on from a PC.
Find out if you can do 1:1 for your DVI input, and things will look just as sharp. If your set doesnt support 1:1 on DVI, you will need to find the correct resolution to avoid any scaling of the image.
Unfortunately, that resolution may appear overscanned on your set.
A lot of the samsung tvs seem to overscan on dvi/hdmi, mine used to as well. When on the dvi source, go into image settings for the tv and see if there's a "Just Scan" mode. That does 1:1 but it's not default. If it doesn't have it there may be an updated firmware for your tv to get it (as there was for mine).
My Sony had the same problem until I discovered Game/Text mode. It enables the required 1:1 pixel mapping and ditches the god awful "Vivid" Colour mode that they use to sell TVs at stores. Maybe your samsung has something similar.