If you are looking for quality you'll need to spend a little bit more. really if you have a Mac Pro you must be using it for something like maybe video or audio work.
Rather than a 2.1 system maybe just get some medium size studio monitors from a "value brand". Maybe a pair of 5" Samson like. I've seen their "robicon5" sell for $199 for b-stock. That is the price point what quality is better then just sqawks and beeps used in video games
OK, you are not making anything with sound in ti, just non-crital playback. So, yes, yu can get away with something cheap. The brands other have said are good, Harmon, Alltec Lansing,... ButYou don't need a 2.1 system they are not the best for music. OK for games and action movie sound tracks. They lack the critical midrange. Go for the largest staero pair you can afford from one of those brands