First off, let me start off by saying that I have owned the 225, 325i, MS2i, HF-1, and have spend a considerable amount of time with the RS2 and RS1.
Personally, I would not jump on the *obvious* upgrade, the 325i(s). Although I love the headphone, and is the only Grado which I ended up keeping, it is definitely a different headphone than the rest of the bunch. There is also the MS2i(s) option, but you will not really get the full Grado sound with that headphone. Although, I believe that it is more Grado-ish than the 325i(s).
People would generally recommend the RS1 in your situation because it is a very good headphone, and the difference in price to sound ratio makes it a more sensible choice over the RS2. I would agree with this, but would not agree that it would be a very reasonable upgrade to the 225. Not because the RS series headphones do not sound much better than the 225, which they do, it is just that the 225 is much more "socially acceptable" lol. Although it makes a lot of noise, it looks somewhat cool and is relatively portable as long as you dont mind hanging them on your neck. When I would wear the 225 or 325i, I would get a lot of comments like "cool headphones!". (I dont take the 325i's with me anymore, because I am have found the wonderful world of custom IEMs.) The RS series on the other hand is not transportable at all. There is no way that I would want to take those out in public. First, I would be worried about messing the wood up, and second I would not want to look like I have a tree hanging around my neck (but your style may differ from mine).
That last leaves the HF series, which is what I would recommend to you. Personally, I believe that the HF-1 was a very good sounding headphone, and a lot of people here would agree with me. Unfortunately though, they are discontinued, and rather hard to find. The HF-2 is now out, and somewhat available (I hear they are out of stock or something?). I have not been keeping up on their sound (those threads are too long for me), but I have no doubt that they sounds very very good. I personally believe that the HF series headphones are a much more practical headphone, and if you want to even consider taking your headphones out of the house, these stomp on the RS series.