I did not hear much differences between the 325is and RS2i, but the RS2i are noticeably lighter and more comfy.
I've never heard the PS500, but IMO, since you have the 325is, I'd consider the PS500 or the RS1i.
I found the Rs1i to be a step up from the 325is, I think because it has larger (more volume) cups. It also has nicer hardware (metal gimbals).
If you search the reviews of the PS500 you will many comparisons to the RS1/2. Enjoy! SFYW!!