An image backup of a whole RAID 0 array is going to be crazily large (and you need to backup everything for the inevitable hdd failure). Better to not even bother with RAID 0 and just go with the original plan of two separate disks and manual backups. Its not worth the risks. And as you said, the performance gains are not that relevant for a desktop system. Server, yes. Media center PC, probably not. Gaming/workstation PC? Not really. The CPU is generally a bigger bottleneck in file I/O than the disks themselves in a single user workstation, where the disk isn't being constantly bombarded by requests from dozens to 100s of concurrent users.
A well-developed Windows install doesn't rely on dozens of registry-bound applications. There are always alternatives that store their settings in local files, with only a minimal amount placed in the registry when you run them. Hell, even our reason for being here has never needed to be reinstalled. My SE4 install dates from the original installation back in 2001, which has spanned two separate computers, a failed primary hdd, and half a dozen other MS OS re-installations (some of which involved oscillating between win2k and xp, so I'm not that crazy
). Even SE5 just asks for a CD key on first run.
As time goes on, I find myself having to reinstall fewer and fewer applications with OS reformats. Other than poorly designed MS and Adobe software (and some games), the stuff I do need to reinstall still tends to store the real app data in local files (or at worst in easy to copy/paste Local Settings folders), so reinitializing them is a cinch. It also helps to point a few key folders, like "My Documents," to a different partition with TweakUI (I'd recommend against Program Files, cause a ton of garbage builds up in there; instead install apps you want to keep in D:\Programs). When I reformat, most of my apps can be run directly from the D: partition without any extra effort. A few others need a backed up settings/profile folder copied into the Local Settings folder, and are then good to go. The 3 or 4 apps that actually need installation can just be installed when needed in the future. No big deal.
Would I be better off with disk images for restoration? Not really, since that doesn't remove the cruft that builds up over time, the primary reason for reinstalling the OS. Frequent disk image backups would only let me undo the changes for the past week or two. Malware infections that can't be fixed from safe mode are few and far between, especially with XP SP2 and Vista (anecdotally, I've suffered twice as many hdd failures as irreparable malware infections). A base disk image of the Windows partition post installation and basic setup would help somewhat, but it would fail to be relevant if I ever change hardware components or decide to switch to some different apps. Such necessitates creating a new base image, which over time results in more time and effort than just reinstalling the OS and restoring the Local Settings folders.
Interestingly, a lot of this is just coincidental to the apps I prefer using for various tasks. It seems that the better software design these apps have overall winds its way all the way to the bottom level choices of where to store critical data. The registry was a terrible invention that just needs to fade into dust already...