Transport for 250 GB of Apple Lossless files that I control over Wi-Fi from my Powerbook G4. NEEDS AN EXTERNAL DAC! Currently connected via optical cable into Denon 3803.
You could pic up an external dac from Scott Nixon Audio for around $200. At this point I think that that would give you the biggest improvement. Maybe replace the speakers with monitors from either paradigm or tyler acoustics (I think they just came out with a cheaper line of speakers call the "freedom line", which are supposedly a great deal) I those two changes will make an incredible difference. If you are still not satisfied after that, I would consider an amp( i can recomend some if you want)