I have the Samsung Galaxy S and have recently updated the sound using SuperCurio's kernel and software and I have to say that the improvement is awesome. Basically it removes the limitations that Samsung have put on the phone in order to adhere with European legislation and also directly accesses the sound chip.
It's not quite perfect - I do still get the occasional pop out of the headphones but when paired with decent top end headphones - I use Triple Fi Pro, Ety ER4P and Shure 530's, all three work brilliantly and produce superb sound quality. Strangely the Ety's have decent bass thump, the Triple Fi's have decent mid and the Shure's have some sparkle to the top end - don't ask me why - I can't explain it.
There's certainly no need for an amp with the phone now. At the moment I have 8gb on-board memory coupled with 16gb in micro SD. I may get a 32gb at some point in the future but at the moment it does meet my needs and transferring music tracks onto the Galaxy is easily as fast as any other portable player out there.
Hope this helps.