Well, depends on how you define stability. If you checked your OC using prime95 on XP (the standard 32bit 25.6 prime), and it was stable, then it should and will be stable on any x64 OS as well.
However, if you only tried crunching on XP 32 vs. a 64bit OS, you might experience instabilities because your setting wasnt really any good in the first place, it just didn't show because the CPU wasn't loaded that hard with 32bit BOINC.