I have acouple of TFT's (not especially High spec, AOC brand so more budget end to have two head up). Anyway I have run them side by side as a two head with one DVI and one VGA - have to confess I have had trouble determining a difference :squareeye :suprised:
In any case I do know this is ONLY relevant to flat panel displays, and that it is really just if you really want the best output you can have on a high spec monitor wher eit may be barely distinguishable (to my eyes anyway) - and that you will pay through the nose for one of these "cool" DVI leads! I saw some going in Maplins last week for £45 (like about $80!) ... that was the gold plated ones..... so I bought the £20 one!
Anyway - it is not a biggie - rather like if you have paid a shed-load of $'s for your huge TFT then spend that bit more and make use of the DVI. Also often you will have option of both inputs being plugged and you can make use of that if you have more than one machine around - to use the one screen for both boxes. For me that is where DVI becomes truly useful. Also in a 2 head (like I said for me) you will need to be using the DVI for one and the VGA for the other, chances are.
Hope I covered it all - since I seem to be in a position to say.