Whats the rule of thumb when buying a 2.1 sound system for your PC?
I don't think there is a rule of thumb. I assume that you are looking for something in the 60 - 200 price range, and there are lots of options. Don't dismiss 'non-PC' brands, i.e. people more focused in the consumer or pro space, rather than being something PC specific. I had (have) a Klipsch Promedia 2.1 system that set a benchmark ~10 years ago, and Logitech (and Creative) have released some decent speakers in the intervening years.
BUT, all of these only sound good with a 'for the price' disclaimer. My current Mini Maggie system makes all these older speakers sound like Fisher Price toys.
At $60 on sale, the Nuforce S-X is a
good deal, but needs an amp.
If you have the space / budget for it, look at something like the AudioEngine or other pro audio powered speakers (Dynaudio, M-Audio, etc.), because these need to provide a certain level of clarity / power.
Just found this Lifehacker link -
http://lifehacker.com/5957537/five-best-desktop-computer-speakers. What makes me laugh is that the #1 pick is the Klipsch speakers I bought 10 years ago. Note that the volume pod has died twice in that time period, and the 2nd time I paid for the replacement out of warranty.