If its running an XP2500 Barton at stock then the board can definitely do 166MHz FSB so you could upgrade it to at least a 3000+ 166FSB chip, check the motherboard make with a program such as si soft sandra or everest and then see if it can support a 400FSB...if it can then your good to go upto any socket A chip, so upto 3200+.
They didnt make an XP 1200. XPs started at XP1500 (1333MHz) and on the 133FSB went upto 2600+...download CPU-z and check whether its an XP or a thunderbird...likewise check what motherboard you have and see what FSB it supports maximum. Some older socket a boards could only support tbird/duron spitfire/morgan chips and system would work happily with a slower CPU, no problem there but it depends what the highest CPU your motherboard will take as to what you can upgrade it to.
hope this helps.
ghgh