The original question discusses the OC-ability of the 750 T-bird versus the undeniably good ability to OC the Duron 850. I have read that the 750 'Bird does a good job at OCing also and like you, I've also read and seen reviews that show that the higher clocked 'Birds (1.1 and 1.2) don't overclock at as high a ratio as the slower 'Birds like the 750MHz because the Thunderbird series is reaching its clock limit because of heat and system constraints.
If your choice is between a 750MHz T-bird and an 850MHz Duron, I would get the T-bird because right now they are exactly the same price, and the 'Bird has more L2 cache.
I would hold off on buying any 266MHz FSB chips until AMD releases the Palomino (1.5+ GHz) later this year--by then, DDR (PC2100) will be more affordable, more useful with real-world applications, and more readily available with a cooler and faster Athlon afforded by the Palomino.

sorry--the post above was not meant to be a new thread, but a reply to a different one.