If you agree with the "all speakers should be identical" school of thought, choose a speaker that's common on the used market and buy 4 to 7 of them. 4 is enough to enjoy 4.0 or 4.1 with a phantom centre. The cheap(er)skate approach is to begin with the best cheap pair you can snag from a yard sale or thrift store. Then buy more through other sources. There is also an argument to be made that rear speakers don't have to match the fronts. Identical speakers sound very different if they're located in different parts of the room. At least in my room. Perhaps a proper home doesn't do that.
For the subwoofer(s), I suggest a sheet or two of MDF from Home Depot, one to four quality drivers (10" to 15"), and a weekend or two making sawdust. There are "plate amps" ready to screw into the side of the box, but a secondhand hifi or pro amp usually gives more watts for the buck. [Example: The NHT SW3 12" sub, which listed for around $700, contained one NHT1259 woofer which could be bought for about $125. (Sometimes much less with group buys.) The powered version (with an external amp) listed for about $500 more. So, someone who bought two NHT1259s and a $400 used pro amp got about $2400 worth of subwoofage (is that a word?) for an "investment" of $650 plus MDF and decking screws and time.] If you have a spare receiver, integrated amp, or basic amp already, try that first. With the sensitivity of two (or more) subs, as little as 50 watts per sub may be quite satisfactory. Anyway, the NHT1259 is gone, but there's various current drivers that offer even better value. Check the DIY section of avsforums, kits & plans from Creative Sound Solutions, Madisound, Zalytron, Parts Express. (A few car subs also make good home subs, but they're usually overpriced to pay for all the marketing.)
