Actual output power vs claimed power can make a big difference though. A decent external amp rated at 100x5 is likely to produce more real world power than a receiver rated at 100w (or more), simply because amps are built to do one thing and they are usually better at it. It depends on the quality of each, of course.
My receiver was more than $1k, but the model below it didn't have what I was looking for. When I upgraded to low impedance speakers, even that receiver wasn't enough to handle the tougher load. Had I known that was my eventual path, I would have gone with the lower model receiver and a solid amp, which would have cost me about the same amount but yeilded more power.
You really only need to consider adding an amp if your receiver is strugging with a very large room or if you have tough to drive speakers. Otherwise, external amplification isn't realy a benefit beyond being able to turn it up louder. Louder is not the only benefit of plenty of power. Good clean power can help keep the speakers controlled better, giving you a cleaner, more clear presentation - again, this will only be apparent if your current receiver doesn't have enough power to drive your speakers at the level you desire, in your particular room.
