Unfortunately, whereas the Federal Trade Commission has laid out rules for measuring RMS, there are no such rules for measuring P.M.P.O., so all bets are off on these ratings. Between manufacturers, you really can't compare them, and they are often pretty much meaningless. Between models of one manufacturer, they probably are good for comparison. An inexpensive general purpose speaker system will likely only have one speaker per side. It most likely will not reproduce low frequencies or high frequencies terribly well, so don't expect great music out of them.
Larger systems are likely to have a little better low frequency response.
Higher power ratings may indicate a little better sound, but you can only expect so much from a single small speaker. The next step up is a system with two speakers per side, a larger woofer to reproduce low frequencies, and a smaller tweeter to reproduce the highs. Generally, the larger the woofer and the smaller the tweeter, the better it will sound. If you want good musical capabilities, consider something with separate woofer and tweeter. The best systems these days are generally the sub and satellite systems.