I've been in the process of moving my SL machine (and main blogging machine) from a gracefully-aging H-P mini-tower to an iMac.
The H-P was doing okay in most tasks, with the exception of the Second Life client. On good days I would get somewhere around 12 fps; on bad days the number would be in the single digits. I'm not quite sure why this was the case, because the machine itself hadn't changed, but perhaps the clients have become more demanding over the years. In any event, the iMac regularly gets frame rates in the low 20s, which makes the whole experience so much better. (And thanks to Mr. TriloByte Zanzibar for his blog entry on the new iMacs, which not only convinced me to make the jump but saved me several hundred dollars, as he (correctly) said that all of the models would be fine running SL.)
I have changed Difference Engines on a number of occasions - often enough that I sometimes think I started out with Mr. Babbage's original Engine - but this is only the second time I've tried moving things across platforms, and the first one doesn't really count.
Back in 1985 or so, I had my very first Engine, a beige box of a Macintosh. It crashed a lot, required endless disk swapping (as it had only one floppy drive - never mind that external floppy in the picture above!), and was slow as molasses, but it got me through college. For graduate school, however, I needed something a little more current, a little more compatible, and, preferably, a little more portable, so I bought an absurdly-expensive Toshiba laptop (the T5100, if I recall correctly, a 386 CPU with a gas plasma display, which gave the screen that red glow) (back when one could call them laptops without the liability lawyers descending). It was a nice machine, and I did a fair amount of work on it, until it was stolen one day. Switching from the Macintosh OS to DOS wasn't such an ordeal, as I had only a handful of files, all on 3.5" floppies. Boom, done.
This time around, I had years of detritus, including a staggering amount of crud in my "Documents" folder, a whopping 156 GB of stuff in my (pre-iTunes) music folder - God knows how much of it is in iTunes at this moment and how much of it isn't - and 4-5 GB of pictures, many of them SL-related. I've had two Engines running far more than I wanted to in an already-hot room. Add to that the hassle of setting up a new backup routine, and... this was a lot more fun when I was younger.
In any event, I hope to get things sorted out sooner rather than later.