If you already have a decent mini-to-mini cable consider a SendStation Line Out USB. I like it because it lets me use a short, quality Cardas mini-to-mini cable with my portable amp, and then use a longer (and cheaper) mini-to-mini cable to connect my iPod to my car stereo.
SendStation - Products - PocketDock Line Out USB
At $30 it's not dirt cheap, but it serves multiple purposes for me and it clearly sounds better than amping from the headphone jack. That said, I can't offer a SQ comparison to some of the LODs listed here.