I'm not an expert, but I'm using the DVI output from my video card to the DVI input on my LCD. I notice the picture looks sharper than when i used the VGA inputs.

From regular usage, the differences aren't that big, but like you mentioned with the new HD-DVD/Blu Ray drives coming out, the increased bandwidth of DVI will probably make a positive difference in HD video quality.

I'm guessing a bunch of things affect HD video quality when played on the computer as well... which type of CPU, video card, LCD monitor, etc