If you want to stick with the Grado line and you enjoy the brightness of the SR125, then the 325i is indeed something you should look at very seriously. It will also provide the most marked improvement in SQ over the 125, without jumping up to the RS series. Oh, and if you like bass, the 325i will not disappoint!
To put it in perspective, while the SR225 is indeed a relative upgrade, the most noticeable difference is in it's presentation (warmer over the 125's sound) as opposed to any kind of drastically noticeable increase in overall quality. The 325i is a substantial headphone, just not my cup of tea. Too bright for my tastes.
[EDIT] Incidentally, be wary of any possible recommendations for the HF-1. Great headphone, but bang-for-buck performance has LONG since gone out the window - they fetch a hefty price due to their limited run.