|Sempron 2800+ (166 x 12) 2ghz, need ddr 333|
athlon xp 2600+ (166 x 11.5) 1.9ghz, need ddr 333.
May be the board doesn't support 333 FSB CPU, better check CPU support list.
Try to lower sempron FSB to 133 and memory to 133 aka ddr 266.
We can not fight new wars with old weapons, let he who desires peace prepare for war - PROPHET.