And What About The Others ... ?
Well, what about the AMD K6 and the Cyrix/IBM 6x86MX ? If you let me start with the 6x86MX , I have to tell you that especially Cyrix is selling their 6x86MX chips pretty close to the margins, as well known from the past already. They came up with the - with all due respect - crazy idea of selling parts with different specs under the same name. You can get PR166 CPUs at 2x66 and 2.5x60 MHz, PR200 CPUs at 2x75 or 2.5x66 MHz and PR233 CPUs at 2.5x75 or 2x83 MHz. These CPUs with the same P-rating are actually not performing equally, but who cares? Now I came across cases where you couldn't even clock a PR200 CPU at 2x75 MHz to 2.5 x 66 MHz, which is slightly faster. Hence forget about overclocking unless you got a nice PR233 at 2.5 x 75 MHz, since then you might have a chance. IBM is unfortunately under the threat of conformity with Cyrix and rumors say that IBM is selling their chips the same way against their own believe ... well, well.
This leaves AMD's K6 as the only real overclocking champ and as long as your K6 is real, you've got a good chance to overclock which ever way you like, either with multiplier increase or with higher bus speeds or with both. However there's a little problem. AMD doesn't seem to care much about the fact that remarking their chips can be done by a six year old child. There's only some paint printed on the aluminium heat sink attachment, which you can wipe off with several different chemicals. Printing a new marking on the 'naked' CPU is not too hard to do. This makes a K6 233 out of a K6 200 or even K6 166. Be careful with that, examine the printing very carefully. If it's done well, you won't have a chance though. The incidence of remarked K6 CPUs is cerainly higher than you think, and most reports about 'unoverclockable' K6 CPUs are due to remarked 166 version, marked to a K6 200.