One thing I've noticed is that computer speakers are pretty important because you end up doing a lot of listening at your computer...I do at least.
If you wanted to go with that subwoofer, what you would do is get an 1/8" (headphone) in to 1/4 RCA (stereo RCAs) conversion cable, and then run some RCA's into the inputs of the subwoofer. You don't need RCA outs from your computer, but RCA out sound cards are available (I have a M-Audio one actually) if you want to swap your sound card out.
That subwoofer has a high pass filter in it, so you can get some active (powered) speakers and connect the high-pass out on the back of the subwoofer into the inputs of the active speakers. That subwoofer won't power the speakers of course, so, you'll need speakers with internal amplifiers. There are lots of good options.
Depending on your budget is the route to go. Based on you buying a sub 100 dollar subwoofer, I'm going to assume your budget is conservative.
For 150 bucks you can get M-Audio BX-5's, which I think is a bangin' deal.
http://www.bhphotovideo.com/c/product/386231-REG/M_Audio_9900_50785_00_BX_5A_Studio_Reference.html
The little desktop DX4's are tempting at even less, but I think the BX's are worth the extra bit of cash.
If you are willing to spend a bit more cash, I'm picking up a pair of the new active Bag End PM6 monitors which are about 650-700 bucks depending where you buy it. I plan on posting impressions, but remind me if I don't.
Lots of other options, if you narrow down your budget we'll figure something out.