It seems it is possible to run some games at 420fsb, regardless of Orthos blend failing at 400. Basically you can pick your own poison. As a generalisation, if you edit lots of large files, have important data etc, you will probably want to stick nearer the 400fsb mark. If you play games and just use your system for surfing the web and don't mind the risk of a lock up every now and again, you can go for 420 fsb or so with an e6600 at a vcore of around 1.61 in bios. Vdimm between 2.25-2.30 works well with the team group ram at the 420fsb mark.
The heavier games like Oblivion will need testing by the user themselves to find an acceptable point. Generally Oblivion can knock around 10fsb off an overclock that is not Orthos blend stable, but is stable enough for most other games...
regards
Raja