In my personal experience, the Grado SR-60 does not need an amp. Actually, I don't think you really need an amp with an iPod at all and I've been quite happy with the iPod headphone out with my etymotic er4p's ever since I decided to completely downgrade to a portable setup. If I were you, I'd completely skip out on thinking about getting a portable amp.
EDIT: Well you beat me to the post, so I guess I'll try to answer you question. I guess it depends on what portable amp you're looking for, but think about diminishing returns. In my experience (and to my ears), there doesn't seem to be too much difference between headphone amps as you spend more and more on them, and I feel much of what people hear is basically placebo. Are you willing to dish out $350 bucks for a portable amp that really doesn't make that much of a difference? I guess that's up to you to decide, but I'd skip out on a portable amp period. Like I said, I use the headphone out of my iPod and it sound just fine to me. I think spending a shit ton of money on a portable amp and dock lineout for the iPod is pretty silly.