I suggest you keep looking for a monitor identical to the one you already have. I once tried different monitors, and it did not work out. If the sizes are different, you have troubles dragging windows from one to the other; parts get obscured or get undersized. Even the same size monitor will have different color characteristics which can be annoying.
Be aware that on a 5850, the third port is a HDMI port that requires an active( not passive) hdmi to dvi conversion unit. They used to be $100, but I understand that the prices of such converters have come down a lot.