Most who have experience with S939 dual-cores will tell you Manchesters require lots of VCore to get it over 2.5-2.6GHz. I've got an X2 3800+ Manchester running at 2.5GHz, 1.42V VCore, in my office machine. To be honest, I've not pushed it beyond its current OC (I don't need any more for its role), so I can't speak to how much more it has got nor how much VCore it can tolerate before temperatures go really crazy.
But, just as a comparison, consider that I've got an Opteron 170 (Denmark core, which is identical in all but name to a Toledo X2 dual-core) running in my sig rig. It's sitting comfortably at 2.8GHz, at a VCore of 1.37V (drops to 1.34V at max load). Clearly, the Denmark/Toledo requires much less VCore than a Manchester to run high overclocks. It's a good illustration of the prevalent trends when comparing a Manchester dual-core to a Toledo/Denmark dual-core.