How does an Intel Atom N270 processor compare to an AMD Sempron 2400+?
Atom N270 would be better. Although their clock speeds are comparable, Atom is using a higher 533mHz FSB (vs 333mHz for Sempron) and L2 cache size is double that of the Sempron. If Hyper-threading is enabled on the Atom, the Sempron will simply be left behind on dual threaded programs. And some more on the plus side, the ATOM supports chipset for faster ddr2 RAM, supports execute disable bit and it consumes very little power.
