The Behringer speaker above is quite unusual compared to other speakers. In general, it compares more closely to speakers costing 3-4x it's cost, in both terms of actual quality of build/parts used and in terms of it's measured response. As for subs.... you have no excuse to hold off.... in fact quite the opposite... you should be compelled. Right now Parts Express is selling both it's highly regarded budget subs, the 10" and 12" powered units, for nearly half price. A pair of the 10" ones would cost you only $160! You will get better sound using a pair of subs(one on each side of room), especially with the smaller 2 way speakers used all way around. Having dual subs allows the 80-90Hz optimal xover point to be a seamless transition, for music and movies, to the small speakers, in effect giving you '3 way' speakers.
-Chris