The problem is the Bandwidth of the DVI connectors, which is based on the TDMS used. Single allows a max of 60hz usually @ 1600x1200, but sometimes up to 1920x1440 if a rare high end chip. Usually to push above 16x12@60hz you need dual TMDS on a dual capable cable.
Most gaming cards will usually max out the bandwidth of the DVI connector @ 16x12x60hz with their single TMDS. But look at this review and see how cheaper and slower parts quickly restrict your resolution.
<A HREF="http://www.extremetech.com/article2/0,3973,1370500,00.asp" target="_new">A Good Comparison of DVI Quality by ExtremeTech</A>, which also should give you a little bit better understanding of DVI.
The VGA connector can be better but requires more tweaking usually and alot more money and gear. For the average user the DVI will give you truer colour and better picture. However, it really depends on what you find works best with your setup, nothing is a sure thing for either.
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