Powersweeps work great for the first couple of weeks, and then you run into problems. I had one in my 55 gallon and could never get it to stay on the glass. It's so heavy towards the front of the pump that it pulls the suction cups off the glass. I replaced the suction cups several times, but could never get them to hold.
I also had problems with mine not sweeping. Like I said, it worked well for a couple of weeks, but quite shortly after. I disassembled it several times and soaked it in vinegar, but could never get it to work again.
I've since moved to maxi jets and have been very satisfied. I currently have 4 1200's in my 210, and am impressed with the dependability and water movement they put out.
To answer your ? about how many powerheads you need: it all depends on the amount of flow your return pump is running, and what type of corals you would like to keep in there. Opinions vary, but I would try to turn your tank over anywhere between 15-20 times an hour.
The other good thing about the maxi's is that they put out very little heat.