I just got a Logitech Z623 and I love it. The sound quality is blowing me away for the price, and I am not foreign to good speakers (I have a set of Polk RTi's).
If you can stretch the budget by $50 the following combo is pretty killer.
Refurb Audioengine A2+
Polk PSW10
The A2+ have dual sub outputs and it also comes with a USB DAC built-in for your PC. No PC speaker will come close to the SQ of this combo.
hmmm not too bad, that sub is huge though, I couldn't fit my legs under my desk if I went that route
Ohh damn. There isn't another place you can put it? BTW the sub is now $20 cheaper at Amazon which makes it an even better deal. It's front firing so placement is not as finicky.
Klipsch Promedia 2.1s are under that price, they were always a well regarded 2.1 comp set. I loved mine when I had them
http://www.amazon.com/Klipsch-ProMed.../dp/B000062VUO