Check out this article:
http://www.ecoustics.com/electronics/products/articles/131062.html
It gives you a bit of a rundown on the "frequency response" spec and shows a couple of hypothetical frequency response graphs. The published spec is basically complete BS (although it does help to determine how low the useful bass goes). What the guy doesn't tell you is that your room affects the curve even more than the actual speaker does. Moving your speaker 1 foot to the left can change its response curve at your listening position substantially. Having a "live" room with lots of hard surfaces will wreak havoc on the sound, as will having a too-"dead" room with lots of carpets, curtains, and other sound-absorbing furnishings. There are all kinds of treatments to correct room problems, but suffice it to say that the average home speaker is designed for the average home, and if you put it into a room with 2 walls full of floor-to-ceiling windows then they will not sound as intended.
So this is the reason for buying a receiver with MultEQ calibration. It takes actual measurements in your actual room with your actual speakers, and averages them over multiple listening positions to correct the sound. The frequency response of the speakers becomes a minor part of the overall equation.
Of course, this isn't to say that speaker quality should be ignored. Distortion happens when speakers are pushed too hard, and crap drivers will distort quickly and often. Crap designs can also make for peaks or valleys in the response curve that can become uncorrectable. IMHO a 2-way is harder to mess up than a 3-way. They are simply less complicated. All of the speakers I've ever owned have been 2-ways, and I have never found them wanting.
Size is another issue. There are two competing factors at work. First is the front baffle size; where the drivers are (usually) mounted. In general, a narrower baffle will give the sound better dispersion and phase characteristics. However, in general you want larger drivers for the bass, and they simply can't fit in a narrow baffle; and larger size of the speakers overall will also give better bass response.
In summary, acoustics is supremely analog. You can't apply your usual digital purchasing decision methods.
The Onkyo 60x series is a great value and my typical recommendation for most people. I also usually recommend the Polk Monitor or R series; towers in the front, bookshelves in the rear, center in the center :hmm:. They are a fantastic value at their "sale" price at Newegg or Fry's. Also, they have something which is stupidly missing from most speakers makers' lineups: continuity. Polk has made similar speakers using similar drivers for over a decade. Buying a new pair to aurally match to your existing ones isn't a problem, even if your existing ones are 5 years old.
Spend about $500 on the speakers and you'll be happy. If you can't afford that right away, just do the front towers and buy the rest later. Or, perhaps even better, buy a sub and the pair of bookshelf speakers that you will eventually use for rears; use those speakers as fronts until you can afford to buy the towers and center, and then move them to the rear.