The watt rating of a speaker is not a very important figure really. You aren't underpowering with less than 400 watts, it basically just means you aren't blowing it up.
And here's a little lesson on how companies lie about wattage ratings for receivers:
Your sony mini-system is rated thusly:
130 Watts Total = 65 Watts @ 6 ohms per Channel x 2 (10% THD, 120 Hz - 10 kHz)
First of all, you don't get to double the wattage just because there are two speakers, so right away that means the true wattage rating is no more than 65 watts.
Second, it's rated at 10% THD - which is a distortion rating. A more usual quality standard is 0.1%. That means that this is being rated at a level with 100 times as much distortion in the sound as is usually considered acceptable.
Third it's rated for 6ohm speakers. Some speakers are 6ohms but the more common standard for most ratings is 8ohms. This also inflates the apparent power of the receiver.
I'd be very shocked indeed, if rated by usual standards for audio equipment, if that receiver was rated more than 30 watts.
Which, by the way, would probably drive your speakers fine if you aren't playing very dynamic orchestral music and/or trying to fill a large room. However, almost any "real" receiver would be a big step up in power.
