What's the fastest AMD cpu I can use in my socket A board

The quick answer is take the maximum speed of your FSB and multiply it by 20.5. For most of you, this will mean they haven't made a cpu that you can't run in your board. I don't care who made the board or what model it is, this is the only thing that matters, barring initial design flaws by the manufacturer. The one thing that you can't do is use the bogus FSB numbers. Example: the FSB of the 2800+ is 166MHz, NOT 333MHz. The official minimum FSB speed for all K7 CPU's is 50MHz and the maximum is 166Mhz. Although both of these numbers can be exceeded in either direction.
So if you have a 1st generation KT133 (non-A) chipset, then the fastest you'll get your FSB to is about 116MHz. this would limit you to 2378MHz. That would equate to approximately the speed of a Athlon XP 3000+.

I believe all the other information that is required to use these faster cpu's is contained in the other cpu articles on this wedsite. Read them and you should have a good understanding of how this works. And be sure and follow the links. They provide excellent info.

Updated: 4/04/03
It's now known that the top multiplier is 24x. This was discovered with the introduction of the B core 2100+. Several people (including myself now) have installed this cpu in a KT133 chipset board (Abit KT7's) and it booted without modification at 2000MHz default (20x100). Also see multiplier cross reference on this site.

Update 12/05/03: CPU's with a date code of 0339 and later are being multiplier locked internally by AMD. Since the multiplier can't be changed via any method at this time on these locked cpu's, you will only be able to run the cpu at default multiplier times the max FSB speed you can get from your motherboard.