It's really up to personal preference. A $20 system without subwoofer won't have much punch, and a $20 system with subwoofer will sound muddy when turned up to high volume.
I have the Logitech X-540 5.1 system but with the way my setup is I only use it as 2.1 It gets the job done pretty well. But I really prefer to use headphones, even cheap ones. IMO the stereo separation and directional positioning is much better with headphones.