Personally, I'm not a big fan of RAID0, since I'm running one right now.  The primary slowdown factor in HDDs is seek time, and RAID0 doesn't really improve on that.  Also, if you're using an onboard RAID controller, it's typically not a "true" RAID => you need drivers to run the thing, which slightly taxes your CPU a bit.  Also it's more difficult to install some OSes using a software RAID.  There's also the possibility of a drive failing, but I really don't think that occurs terribly often on a personal computer.

Having a separate HDD for Windows is interesting, though a partition should be able to do the job too.  As for reliability, it's a little debatable I guess...