I have tried that also. I actually just let it rest for 10 minutes or so then tried again. I read somewhere that it has to be "root" with lowercase so I tried that even though I have tried before. Weirdly enough, it actually worked this time.
HDZ955FBGMBOX is C3 revision
HDZ955FBGIBOX is C2 revision
You can EASILY overclock the 955 as I have to 3.6 without having to try hard. No increase of voltage required. You can increase the multiplier or the front bus, I chose front bus to get my RAM faster.
I have joined Lockerz and and not really sure if it's really legit. I mean, if you haven't really gotten a prize from them yourself, I wouldn't really advertise such a site. But it really does seem legit.