DEAFisherman, The 3 units you mentioned are basically the same, 2.2kw, 16 mile radar units, Furuno's entry-level radar. The 1621 was replaced by the 1622 and last year (2003) the 1623 replaced the 1622. The 1623 had the same specs as the 1622 with a new keypad and $150 added to the price tag.
I think BW has them for $1150, it will be on sale for around $1000 at the boatshow.