So far I haven't seen the OP state he wants to "fill the room" with sound.
It's not *just* that, as I've qualified above. The output power has a lot to do with creating a quality sound from the speaker. Poorly powered speakers can sound murky.
For near field monitors with the selected amp they *will* produce sound. Low end extension from just about any desktop speakers is going to be lacking. For that, a subwoofer would be required.
Most people want a full range sound, so yes a sub is usually desired in a desktop setup due to lack of space for floor standing full range speakers. People usually listen to music, videos, and play games on their computers. All of which contain a dynamic range of sounds/noises. Full range speakers are cost prohibitive here anyway.
As I also mentioned above, there's plenty of 2.1 PC speakers available that will come in well under the $250 projected budget that would likely offer better overall playback flexibility. I really don't feel that you'll find a better balanced 2.1 setup for $150:
http://www.bestbuy.com/site/klipsch...lack/9382232.p?id=1218095770265&skuId=9382232
Most of the newer onboard sound cards/chips are decent enough for most uses. I don't think a dedicated DAC is really going to buy a ton more clarity, so my conclusion: get decent speakers first, then see if you feel like the sound card is holding you back. The odds are the sound card is sending an adequate signal, and the most gains to be made will be in the quality of the playback device (speakers).