Yep, AMD Sempron is the new replacement for the duron, It`s basically an XP core with a lower mhz rating but a higher (FSB) bus speed.
Older XP`s (Thunderbird core, I think) used to run at 266mhz FSB. The new Sempron (Duron replacement) run at 333mhz and 400mhz FSB.
From what i`ve seen/heard, A Sempron 2200+ (or was it 2400+) Is equivalent to the old XP 1800+ 266mhz FSB speed wise.
Trooper