Hey the sr60s are great entry level cans, good first purchase. But allow me to offer some practical advice, you don't need to go and re-rip your entire music collection into flac. Contrary to popular thinking more bits does not necessarilty mean better sound; modern mp3/mp4 encoders (lame mp3 or even itunes aac) sound transparent (same as lossless) at surprisingly low bit-rate (ok, you might hear some difference if you are using some high end dac/amp and flagship cans, but probably not the sr60s). Any difference you hear between flac and decently ripped mp3 is probably psychosomatic, if you don't believe me try an ABX test in foobar. It's nice to have things in flac to archive, but it's overkill imo and will be a total waste on something like a sansa clip.
Some more advice is don't expect them to blow you away straight out of the box, imo burn in time is important for grados, not only does the sound improve, but you also need some time for your ears to adjust to the sound. I've had a pair of SR80s for about 4 years now, and everytime I come back to them they seem to be getting better and better, like a fine wine that improves with the more wine you drink :D.
If you are on a tight budget, a dac and amp if quite expensive, there are some budget options available, low-end nuforce and fiio do some nice entry level amp/dac combos, which would probably be the best way to get the most out of your cans; but I wouldn't say it's necessary yet, the grados are quite easy to drive, and you will get along fine without a head-amp for a while. I also wouldn't bother modding your grados yet, especially if they are your first cans, things can go wrong and you could be left headphoneless! Saying that though, you might want to consider changing the pads, as quite a few people find them uncomfortable.
But anyway, enjoy your headphones, everyone remembers their first pair of good cans. x
This post is great. I agree 100% with your first paragraph. If you're ripping your own personal collection you should be doing it in FLAC, no arguments there, but find a program, I recommend dBpoweramp, get the LAME encoders and convert your FLAC to something smaller for portable use. Not only will you be saving space on your DAP, but you won't notice the difference at all. For portable use many people find the LAME VBR setting of V2 to be completely adequate, with the Grado SR60i V2 would probably even be over kill due to the lack of isolation.