The JBL Northridge E series has some decent speakers that are pretty inexpensive around the $200 buck mark other wise you could try to look on
eBay for stuff, or build your own. But keeping the price around $200 can get hard especially if you build your own. Ascendant Audio has some good Home Audio stuff out now if you are interested in building your own and the R&D department for GG Audio (LOL inside joke) is currently designing and testing a Home Theater main that will be close to the $250 ish mark. Now this part is going to get complicated. If you run out of your computer directly you only have 2 channels Left and Right, so if you "Y" both channels together for a sub then you pollute the Left Right separation for the highs and now have a mono system. Or you could only Y one channel and run the risk of some low end being in stereo and only getting one channel. Plus using that amp you are going to need some sort of Active Crossover to reduce the high end. You are prolly going to need to get some type of pre-amp or active cross over system do to this up right before all is said and done. The best way to think of how to do a system is linearly from point A to point B. So, you are driving the system with your computer, which has 2 outputs L&R. They will run into your high amp for your left and right. Easy no problem there, but now you want to hook up the sub. You hook the left and right together since you only have one sub and one sub amp (which you would still do even if you had more than one sub) and now you have a problem. By simply Ying them together you now crossed the L&R together, so you can’t do that unless you are happy with mono-stereo (mono out of 2 speakers). So, you could use a splitter, an active cross-over, or a pre-amp to solve this problem. If you used the Splitter you would still need to get something to cross over the frequency for the sub, and the splitter would have to be 2 channels. With the active cross-over you would need a 2 channel unit, then y the 2 outputs together for the input of the sub amp. Or you could find a decent but cheap receiver with a pre-out for the sub use that to drive the sub amp and use the amp in the receiver to drive the main stereo channels. I don’t know if this helps but I think it makes it easier if you look at the whole system and try to figure out the best and cheapest way to make it work…