Yeah i thought it was far fetched but an external sound card for a laptop would cost a bomb.
The theory is that an internal sound card works in a "noisy" environment, full of massive amounts of electrical interference. Not exactly the best place to be doing sensitive stuff like DAC. So even a cheap external unit has a theoretical advantage over an external unit.
There
are external DACs that cost a bomb. Most of them are aimed at either audiophiles or professionals. Headroom has a range of combo DAC/headphone amplifier that cost anywhere from $800 to $2,000. The top of the line got a
very good review from Stereophile. Pro units usually do both DAC and ADC, and are designed for people who need to do mobile recording and mixing.
I personally think the better deal would come from buying a good CD player that had digital inputs (ideally coax or Toslink, but USB might also be acceptable). That way you can plug your computer directly into your hifi and take advantage of it for DAC, amplification, and speakers. Of course, this assumes that your computer has digital audio outputs, which most laptops don't.
And since mine are mostly 320mp3 & Apple lossless, i was hoping it would help to refine the sound further and not to restore details.
You might benefit from a decent external DAC, but don't spring for ones with fancy features like those from Creative or Soundblaster. Those are designed for people who listen to crappy 128kbps files or RM streams - stuff that's so crappy to begin with that even some simple EQ adjustments might make them sound better. I'd personally start from a simple no-frills external unit (you should be able to find them for about $20). Then step gradually up. Don't expect miracles though, since the quality of what gets to your ears is limited by the quality of your weakest component.
Update: The
Benchmark DAC-1 has had some rave reviews, and
Stereophile has used it as a benchmark for some years. It costs about $1,000, which is pretty reasonable considering that Stereophile claims it very handily competes with the
Mark Levinson 30.6 - an absurd piece of audio engineering that costs $17,000. This review also identifies some cheaper <$1,000 choices.