You may be hard pressed to find a receiver in that price range that can push a clean 400 watts or whatever the root mean square is. I couldn't find it in the specs but I'm assuming they run at 8 ohms. I love the Harman/Kardon avr254 and avr354 because they can decode the new master audio and trueHD. They even have 2 power amps inside!!! I own the avr247. They all only push around 75w, BUT its at between .07-.09 THD. So it's somewhat subjective.
The most important thing considering your budget is to decide what features you want. HDMI? bluray audio decoding? sheer power? 2.1, 5.1, 7.1? Movies or music or both?
If it were me, and I had to do it all again...I would get something with HDMI 1.3, with a really good surround processor, forgetting all about the power rating. You can always go out later and buy:
1) An additional receiver that boasts an awesomely clear power amp but a shameful surround processor. These are ALWAYS on craigslist b/c ppl don't realize why their receiver sounds so bad.
2) A multichannel power amp. This will rock your world. The beauty of these is that you can get an amp with 5 or 7 (or whatever) seperate amplifier blocks. This allows more accurate control of the drivers in your Polks. What this does is actually cause your speakers a longer throw, creating more air displacement with less heat (heat is bad, heat is the end product of inefficiency). I would venture to say this is the largest deciding factor and biggest road block to good sound.
It's usually cheaper and less of a headache to upgrade power specs than processing and I/O's.
One last thing to consider is that you just bought speakers with a fairly high power rating so I'm sure you're just itching to blast them. The volume of your system is most dependent upon your amplifier's power rating, not your speakers'. If you by an underrated receiver/amp and crank it up to your speakers' expectations, you would most likely be sending a distorted signal to the speakers, which would damage them. You could ruin a pair of 1,000 watt speakers by hooking them up to 100 watt receiver and turning it all the way up. It would actually be less harmful to do this with a 500 watt receiver, dB for dB. This is headroom at its best, use it to your advantage.
I hope you find this more helpful in your decision making process than "get this one" with no explanation to back it up. If that's what you wanted though, my brother has similar Polk towers and had great results with both yamaha and onkyo products.