I'm a bit of a Gradohead. I've owned, at various points, the SR60, SR225, SR325i, HF-1, HF-2, three different pairs of RS-1 (original "buttoned" cups, buttonless cups, and recabled/balanced original "buttoned" cups) two different pairs of GS1000 (one recabled, one stock), HP-2, and currently the PS1000, the only can I currently own. Grado's do benefit from a better headphone amp but since they are low impedance they don't REQUIRE a headphone amp to sound good, like some other cans (the sennheiser HD650 for example really needs a quality headphone amp to sound acceptable). Having said that they do sound better with better amplification, but you don't have to worry about that right away. I always recommend the same Grados at various price points.
SR60 for the budget can, the so-called gateway Grado
SR225 is the next step up, and probably the best over all Grado value
then I skip straight to the RS-1. its a true classic and the most iconic on the current production Grados
The best Grado ever made is the Joseph Grado HP-2/HP-1000/HP-1. Hell it might be the best dynamic can ever made. It has gotten me closer to music then almost any other can. However they are expensive, out of production, and the only person on the planet who can repair them is Joseph Grado himself, and he is getting up there age-wise. I sold them because owning expensive cans (think $1300-1800) that are out of production and not easily repairable made me nervous.
I can't recommend the GS1000. It is an interesting can and very comfortable but it is devoid of any meaningful midrange, and when it comes to Grado if you don't have the magic Grado midrange for that absolutely perfect crunchy electric guitar whats the point. The PS1000 brought the midrange back into the GS1000 type design and I love it, but it is a terrible value. I got mine for waaaay less then MSRP used and am therefore very happy with them.
Whats your budget? if you can swing it snag the RS-1.