Some people prefer powered mixers with unpowered speakers, some people prefer mixers with separate power amps and unpowered speakers, some people prefer mixers with powered speakers.
With a separate mixer (from amps/speakers) you can always expand. If you have a powered mixer, you are stuck with whatever features it has (# of inputs, output power).
Choose your system based on requirements and any future expansion needed. Those Sennheisers John linked to are neat, but I would never recommend wireless PA speakers except for those situations where running a cable from mixer to speaker is difficult.
What's your budget? What is the size of the venues? Go listen to some speakers at a music store. Can't go wrong with MOST JBLs. If you've got the budget, look at QSC as well. Less money, look at Mackie and Peavey.