A Duron 800 is only slightly slower than a pIII 750. But you can get a Duron 900 for the same price as the pIII 750, making it faster, and with the AMD solution you will have a much better upgrade path. The same motherboard you put your Duron in will support AMD products up to 1.666GHz, provided you get one with support for a 133MHz Front Side Bus. So in a year, when 1.333GHz Athlons are $50 you can cheaply and easily add a big chunk of processing power to your machine.
/Athlon-1.2GHz@1370MHz(137MHz*10)/Asus_A7V133/