Look at the Tekton Mini-Lores ($649/pr.), they will rock with that Onkyo. They are very good floor-standing speakers...
Since you are considering used speakers my suggestion is to buy locally since shipping speakers is expensive and, other than turntables, speakers are the most likely item to get broken in shipping. Check local audio outlets, Craiglist, and there is a function here on "my page" that allows you to sort Audiogon ads by locale.
The other benefit is that you will probably be able to hear before buying, and you have given no preferences in your posts. This has led to one gent suggesting a low efficiency mini speaker and the other a high efficiency floor stander. Both are right, because you have given no specifics as to what you are looking for.
That said, the tide has gone out on the 401s and pretty much anything in your price range is going to sound better.....but better to you, who knows?
Buy locally and consider some nice high efficiency speakers like Altec and Klipsh, and some more mainstream speakers, B&W, Paradigm, Snell, etc. If you put in a little time, you will find something that matches your taste and budget. IMHO, for me, a 240 square foot room with high ceilings would require larger, floor standing speakers, but your taste might certainly be otherwise.
Your room is about the size of mine. Are you a bass nut? How loud do you listen? Are you going to be running a sub? Is this going to be a dedicated listening room or a family room/den? How much freedom are you going to have in setting up the speakers? Is your setup going to be on a short wall or a long wall?
If I was you, I would stick to the used market and buy locally. As already mentioned above, there are the Tekton speakers that many are recommending these days, but I really have to wonder how many of those people have actually heard them. All I've done is read the reviews and they have gotten positive reviews, but reading between the lines kind of makes me think that they might be sensitive to placement.
Dollar for dollar, I think that you'll get a better built stand mount speaker than you would get in a floor standing model for the same price. Of course, you'll still have to consider the cost of the stands.
I've been a long time fan of stand mounters because they image very well and sound better for the money spent.
Also beware of speakers that offer a money back guarantee. Most are pretty picky about the condition that the speakers have to be in to get your money back and you'll have to pay shipping both ways, which can be rather expensive for floor standing models.
You'll also have to consider your Onkyo. Do you want to buy speakers that match well with the Onkyo or do you want to buy speakers that will more likely do well when you upgrade? The power rating that you stated for your receiver (if that's what it is) is likely for an 8 ohm load. Do you know it's rating at 4 ohms or whether it can handle a 4 ohm load at all?
I'm only bringing up your present amplification because I don't want to see you get a pair of speakers only to be disappointed with them, when the true problem may be that you aren't feeding them with enough power.
From this perspective, the Tektons may be worth considering because I believe that they are pretty efficient.
In any case, I'm running a pair of Silverline Minuet Supreme Plus bookshelf speakers. My amp is an Aragon 2004. Although it's only rated for 100wpc at 8 ohms, it's a pretty beefy amp and delivers a good amount of power. With this combination, I can get loud enough that I can't hear myself think. The little Silverlines also deliver a good amount of bass.
Some others that might suit your needs, considering your budget, are the Paradigm Studios (10s, 20s or 40s), Dynaudio DM 2/6 or 2/7 or B&W 605. All of these models did well with a variety of music.
Of all that I mentioned, I stuck with the Silverlines. They worked the best for me in my room. They truly do sound much larger than they really are. They are also easy to drive; and on the other end, they can take a lot of power too. They can be had new for $700 and about $400 - $500 on the used market.