Good comments here so far. The only thing I can really add to this discussion is that results will be very, very mixed, and depend on a lot of things - price, the components the amp will be matched with, what level of components you're comparing to, and even your own person preference for sound. For some people adding an amp is a good idea, but for others it could be completely unecessary - other areas might benefit more from being upgraded first.

Mr Peabody's correct in saying that higher end a/v recievers are likely to sound better than a $500 Denon hooked up to an amplifier. If you're spending $600 an a couple of outboard amps you might be better off to consider a higher end receiver. Usually those receivers use most of the same components as their separates, you're paying for that extra refinement. At a $700-$1000 price difference though you'd should expect no less.

Many of the better receivers will put more emphasis in the preamp, especially catering to those who want good stereo playback. Of course, you can only put so much in one box and keep the price at a point, so up the sound quality, and goes a few bells & whistles.
Yep! $500-$600 or so is close to the point where a lot of the common brand name receivers just start to benefit from the trickle down effect from higher end models and take the first step up in improved sound quality. I don't think it's coincidence that this seems to be the level where companies first offer full pre-outs.

FWIW, most a/v receivers I've owned (typically the $800-$1000 price point) have had pre-amp stages that were closer in performance to separates than their internal amplifiers were. Neither were as good, but definitely the amps seem to be where the bigger sacrifices were made for whatever reason. Or my speakers are just more responsive to the amp differences, either way. I'm not sure, but I suspect in the $200-$300 the pre-amps stages wouldn't be worth mating with better external amps.

I like the option of adding amps to a/v receivers simply because it's a more cost-effective solution to achieve certain goals. In my large room, even a 110 watt X 7 receiver runs out of gas and starts to strain at higher levels. When this happens it's easy to hear. I benefit from adding a few power amps there, not just for the amps' improved sound quality, but to keep things sounding good at louder volumes. That and I'm a sucker for buying used amps when I see an irresistable deal. I've got too many as is.

I keep listening to Anthem, Rotel, etc pre/pros trying to convince myself to buy one, but the sound quality benefits I hear aren't as big as I hoped for vs the receivers I look at that I just can't justify the cost difference for what for me is primarily an HT only system. Not only that, there's a few useful features in the mess of gizmos that Denon et al pack in their a/v receivers that a lot of pre-pros don't include, which they should.

I almost bought an Anthem when I bought my Studio's a few years back. Then looked at a Rotel when I added 6th and 7th channels. Decided to buy a good integrated instead and use 2 separate systems. Might have been different if I had to fit everything in 1 room. I suspect in the next year or two I'll be looking at one again to get the latest processing for BluRay or HD-DVD. Maybe I'll find a pre-pro that finally impresses. Maybe I'll stick with a receiver and buy a nice pre-amp and stereo amp instead. In the meantime I have 3 power amps connected to my receiver which was the best way for me to get what I wanted.