Monitors!

[Sumfun]

I am sure I will take heat for this next comment, but in my opinion a great way to go is just to get some cheap speakers, because at least that way you will always have a reality check on the sound quality. Same theory as an NS-10, if it sounds good on the worst, imagine how it will sound on the best!

Okay, here's the heat. Cheap speakers tend to emphasize the midrange, so when your mix is played back on decent speakers, it will sound crappy. Better to use flat monitors so that you can hear everything accurately.

Also wanted to mention that room treatment includes reducing the noise from your PC as much as possible.