I have an ABit KT7 motherboard (not the -A version) which operates at 100/200mhz fsb. I currently have a 800mhz t-bird processor. The maximum processor supported is 1.4Ghz 200fsb, but these are becoming very difficult to get hold of.
My question is, if I were to use an unlocked 266fsb athlon, could I run it at 200fsb and up the multiplier to get a decent speed, and would doing that cause a similar heat problem to overclocking the processor.
ie. A 1.4Ghz 266fsb (ie. 10.5x), running on a 200fsb would give 1.05Ghz. Can I just up the multiplyer to 14x and be back to a 1.4ghx, or is it not that simple?

in theory yes, but its going to depend on the chip. I would say it would work.

this guy did it on a similar kt133 board

http://sysopt.earthweb.com/userreviews/mboards/reviewhtml/AsusTek_A7V.html

there will be no heat issue since the power consumption is mhz dependant not multiplier

check here to know what vcore the new cpu needs since your board will probably default to 1.75v which is likely too high

http://www.amd.com/us-en/assets/content_type/DownloadableAssets/Processor_Recognition_Rev03_ENG.pdf

that board is rated for 100mhz only because it was so unreliable at 133. It may actually work at 133.

hope that it will work. If a board cannot id a cpu it often won't boot.