Well, here is the latest state of affairs on my end on the successes (and numerous failures) OCing the 4870 XOC BE.

I managed to stabilize the last 4870 XOC BE I got. I am currently running at 850/1150 with 1.350V on the core (core voltage obtained through volt-mod) and 1.501V on the memory (stock memory voltage). ATITool is artifact free, 3DMark06 can run all test, including features and batch tests for hours without problems. Crysis benchmark runs without a hitch as well. Still no go on the FurMark. I realized that the reason it crashes is because the VRMs get way to hot. Using Everest I read the VRM temps at the moment when crash occurs in FurMark and they are as hot as hell. In fact, they pass 125C mark when FurMark blows up, which is much too hot. Unfortunately, there is no obvious solution to me at the moment that would do a better job at cooling the VRM. I installed the HR-03 GT which comes with bigger heat sinks than those with the Zalman VF1000 and the added fan blows a bit onto the VRMs. I could mod a fan to blow specifically on the VRM heatsinks, but I do not really care. In games the VRMs never pass 95C, so I am okay with that. It seems that I will be settling on this clock. I can push things to 875/1150 with 1.43V on the core, but I do not see the 25MHz gain to be justifiable for added 0.08V. It's not 950/1200, but I guess it is not bad. Only thing that sucks is that no matter what a core volt-mod is required if you truly want a rock solid stable card.