You guys with the 450 Mhz ceiling are running into something other than the boards. CPU or Memory maxes maybe. My DS3 is a C2 and I can hit 515, my Asus P5B-E is a C1 and does 515 as well....
now I've had 4 other C2Ds that had just miserable overclocking experiences with. One that would NOT do anything over 450 Mhz stable - it was MadMikeSS chip, we always figured it was the board but I took it and put it on my p5b and I hit the same limit. He finally changed to another 6400 and is now doing 490 on the same board.
I can confirm there is no chipset ceiling at 450 mhz with the right CPU and a c2 chipset.
F
These were done on the DS3 -
http://www.3dxtreme.net/other/GB%20DS3/3611-1.425v.jpg
http://www.3dxtreme.net/other/GB%20DS3/3570-1.40v.jpg