I'll explain the 3 DDR types:
PC1600: Industry standard, 100MHz clock spped (200MHz DDR),Often barely beats PC133, not worth it.
PC2100: Industry Standard, 133MHz clock spped (266MHz DDR),outperforms PC133 by 10%, Most oftenly seen
PC2400: Ins't an industry standard, Is PC2100 RAM certified to run at 150MHz (300MHz DDR), so in other words it is for fsb ocers, because as the fsb increases, the memory bus increases also.
Just to note, high quality PC2100 RAM (Crucial for example) will often run at 150MHz also. If you're gonna get PC2400, then I'd just get Crucial PC2100 CL2. They're da best.