Athlon XP 2600+, Thoroughbred core, 2133 MHz.

 

I guess a mobile athlon xp mobile 3000+ would be fastest.
http://www2.newegg.com/Product/Product.asp?Item=N82E16819103482
I guess you'll have to find one in a store you can buy from, I'd guess it'd be about 70 pounds, but that's a guess.

 

Yes, I'd go for mobile CPUs too, but it can't be taken for granted that they'll work with desktop boards, especially old ones.

 

It is a Socket A mother board if that changes anything.

 

According to your mobo manual your current cpu is the fastest it will support.

I have checked for bios updates and the last one released was for the Duron1800+.

So maybe you need to get a new mobo and processor.

Regards Howard