I recently took a step in the direction that you are thinking of going. I have the Sennheisser HD555, ATH M50 and some Alessandro MS-1's (just another Grado) and also the E9/E7 combo. I bought the Senns first, then the M50's. There is a big difference between them being that the Senns are very open and lack bass. The M50's, as you might know, have terrific bass, but can be a little lacking in the mid to highs when compared to the bass. I decided to go the Grado route, and I read that the SR80i's are not worth it over the 60i's, and the SR125 and 225's aren't worth that much more price/performance. But the Alessandro MS-1 offer, from what I've been told, comparable sound to SR125's for less money. I suggest taking a look at them. They are much more fun that the HD555's and I'm thinking of selling them, and will mod the MS-1's to what they call here to an MS-1000. It opens them up to a better soundstage and improved bass which would be good for the electronic music I listen to, but maintain that Grado sound that is great for any acoustic/rock.
I'm starting to think I have a very popular and highly recommended setup for a beginner here. Though the more I listen to the MS-1's the more I hate the Senns. Damn you Head-fi!!