I did a lot of investigating before I bought my retail Prophet II MX card, and besides the differences mentioned in other posts, there seems to be a wider variation in 2D quality among the MX cards than there is in 3D. The 2D quality is reportedly influenced by the filtering circuitry on the cards, and is one area a card manufacturer can cut costs.
That said, the two cards reportedly w/ the best 2D are the Prophet II MX and the Asus V7100 series. Now, we don't buy these kind of cards for their stunning 2D, we buy them for 3D. But, many of us still spend a good deal of our computer time in 2D appls, such as internet use and office applications.