The difference between any LCD phone screen and any AMOLED phone screen is like the difference between LCD and plasma tellies in the home.
LCD doesn't make any light of it's own so has to be backlit and loses some of that light as passes through,hence colours are never fully saturated and blacks are only ever dark grey at best.Plasma tellies on the other hand are basically countless tiny neons that generate their own light giving properly saturated colours and blacks that are black.
Even the very best LCD phone screens are still backlit,hence they are difficult to read in even moderate sunlight.AMOLED is similar to plasma in that it makes its own light,but is countless tiny LEDs instead so again will have better colour saturation and is much better in sunlit conditions.
But........the older HTC Desires only had plain AMOLED (like my Omnia 2 had) and NOT the Super AMOLED that the Galaxy S has.It is still better than any LCD screens I've seen which are now at the limit of how far they can be improved.
Chris.