I have a 51 inch widescreen, and have my cube hooked up via s-video cable, and it looks real good. You will get the best picture using 4x3 mode for games that dont have a widescreen option, however I dont recommend this due to burn in issues.
My TV has four differant types of stretch mode and I am sure most newer models do as well, so try the differant modes and pick the one that is the most appealing to YOUR eyes. In all honesty, they all look good, and any stretching usually only occurs at the edges.
Good luck.