Monks being unable to raise their monk level once they take levels in another class is a needlessly limiting, stupid rule, and has been removed.
Every 3 class levels, the monk may increase any one ability score by +1. This increase is identical to—but in addition to—the ability score increase that all characters gain every four levels.
The Amulet of Mighty Fists has been replaced by a Vest of Unarmed Strikes as a standard SRD magic item. The Vest is treated in all cases the same, except that it only affects Unarmed Strikes, it can be enchanted the same way a melee weapon would (including such enhancements as Defending, Holy, etc), and it costs the same as a weapon would cost for those enchantments (instead of three times!!! the cost).