Well, the results so far - after a very PC intensive day with graphics work, web browsing, video playback and gaming (Fallout 3 and Age of Conan, the latter shut down and restarted numerous times as I've been testing out DX9 and DX10 differences) - is that there has been delightfully little noise from the hard disk, compared to the previous five days I've had this PC
(the first three or four with both SuperFetch and System Restore enabled, which was just horrible).
If there are any performance differences between having SuperFetch enabled and disabled, they are so slight I haven't noticed them yet, and doubt I ever would.