Of course Tony is absolutely right, but for pretty much whatever you might want to run, the JBL Power series is about the best value in quality amps today. I just picked up the 4-channel P80.4 for $145 shipped from ikesound.com. I did have to haggle with them a little to get that price, but no big deal really.
Remember, you can't just go by what the manufacturer claims is the watt output of an amp - many brands (Sony, Pioneer, many others) do not actually put out their rated power. On the other hand, the JBLs have been tested to put out considerably more than their rated power.
This is not to say that there is anything wrong with an overrated amp - just make sure you're not counting on the full rated RMS power if its a cheesy brand.
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It's getting to be ri-god-damn-diculous.