The prices are really low...make sure you're getting from an authorized dealer, or accept the fact that you may not have a warranty.
For a 5.1 system, you'll want to make sure the front speakers match each other sonically, so buying the front 3 at once makes a certain amount of sense.
I can't speak from experience, but I do know that Klipsch subs aren't well-regarded around these parts at all. For the sub-$400 price range, a Hsu STF-2, or even cheaper, one of the Dayton subs, are probably good places to start. Others may think of other options.