While all AMD processors now support 64 bit processing, the Sempron processors are AMD's "value" line, run at somewhat lower clock speeds, and have less L2 cache on chip, making them much less expensive to manufacture while still providing very good performance. (see link 1) The AMD Athlon 64 and 64FX processors (links 2,3) run at higher clock speeds and have more L2 cache, providing excellent performance, but costing much more to make than Sempron.  Athlon 64 X2 is AMD's dual core CPU, providing more stable operation for tasks such as video editing.