All LCDs are digital. The ones with analog inputs just convert the signal to a digital one before using it to display the picture. The graphics card has to convert a digital signal to analog to output an analog signal, so basically you're going from digital to analog and back again. This is pointless, but more importantly, it will probably have some effect on how close to the original generated picture the one displayed is. It's certainly going to be very very close, but there will probably be some difference between an LCD with an analog input and an identical LCD using a digital input.
How large will the difference be? Well.. that depends on how good your graphics card is at converting digital to analog and how good that monitor is at converting an analog signal to a digital one. In general, the difference is noticeable but small.