To me it's an upgrade that's not really worth making, you gain a little bit in speed, detail, and soundstage/imaging, but IMO it's not enough to open things up, change the way you listen to music and take things to the next level. You're looking at refinements to the sound as opposed to a big fundamental improvement.
To use a speaker analogy, upgrading to a 325i from a 125 is kinda like moving one model up in B&W's 600 series speakers, it's a refinement that gives you a little more. A fundamental improvement would be going from the 600 series all the way up the the big 800 series speakers.
Personal thoughts? Save up for the RS-1. It's a fundamental improvement over the SR series, and there should be a fair number of them floating around on the used market.