I know this may sound like a silly question, but I have to find a straight (and factual) answer.
Will an Athlon XP (333Mhz) work on a KT266a motherboard?
I suppose it wont, but I remember the T-birds 266Mhz running on 200Mhz-Bus mobos, right?
I don't think that it would be very clever to try and run an Athlon XP on a KT266a board. I don't know if it is possible (only testing can show you this) but it's sure that you won't be able to run the CPU at its full speed, even if it manages to work properly on your board (for starters, the FSB will be limited to 266MHz). The way prices have gone down lately, you can even get a very good nForce2 board that will get the most out of your Athlon XP (especially if it's a newer CPU running @ 333MHz FSB) for very little money.
A few examples: (prices are from Newegg)
Epox 8RDA+ ... $85
Biostar M7NCDP ... $85