I personally have liked Grado headphones. I have owned the RS-1's and RA-1, SR-325's and SR-80's for portable use. I have also owned Sennheiser HD-600. All excellent headphones. In my experience you can get away without an amp with Grados as they are quite easy to drive but not so with the Sennheisers. I'd recommend the amp though if you really want to hear any headphones at their best. Among the choices you mentioned I think your choice will depend also on the type of music you listen to. The aluminum sound chamber on the 325's does sound quite different than the plastic of the less expensive models. If you listen to classical I'd suggest you go to the RS-2 (I think you can get them used in this price range) as the mohogany sounds better, at least to me, with "acoustic" music. If your budget includes the amp then go cheaper, get and amp then change the 'phones later.
who makes a good set of headphones
I'm in the market to buy a new set of headphones for my stereo system and would like some advice. I have been looking at the Grado 125, 225, 325's. I've heard they are pretty good. I am willing to spend $300-400. My other question concerns headphone amps? Are they necessary, and do they improve the sound just marginally or greatly. Any advice would be appreciated.