In Anandtech's review of the 955, they mention something very odd. Vista 32 allowed an overclock of 4.2GHz, while Vista 64 only allowed for 3.9GHz. Weird.
^Exactly. No one's sure why. This also happens with other OSes. For example, Fedora crashes after about 20-30 minutes (P95) while SUSE runs (Prime 95) fine. Both are x64 versions btw.