I pretty much agree with Americanaudiophile on this. Your ears will tell you more than anything. While speakers can vary, most today are 8 ohms, so you should be fine with whatever sounds good. You can pay as much or as little as you want, but if a $50 speaker sounds as good to you as a $500 speaker, go ahead and buy the $50 ones. In most cases you won't really need extra efficiency, but without knowing your taste in music, it's hard to advise. Anything short of reproducing a live Rock Concert should be fine.
And remember, you only have one set of ears. If you have to raise your voice to talk over it, it's probably too loud.
One more note on speaker impedance. The number is always a 'nominal' figure. impedance varies by frequency and an 8-ohm speaker can really vary from 2 to 20 ohms. The problem comes in when you start driving heavy, especially bass. The output transistors (or whatever) tend to convert more of the power into heat at lower impedance, which is usually in the lower frequencies.
Ask for a frequency response graph for the speaker before you buy, and maybe an impedance graph, as well. If they can give you one, it's probably a decent speaker. And check the company's website for company history. But the sound should override most other factors.
Update(6/19): The different power (watts) numbers are based on different testing guidelines. The power supply of the receiver can put out more power into just one or two channels than when all five are driven. Also, wbhen they say at 1khz, they mean it was tested with a single frequency (note on a keyboard). It is harder for an amplifier to reproduce multiple notes without distortion (THD=Total Harminic Distortion, the interaction between multiple notes).