Sempron is AMD’s low end series of processors for both Socket 462 (A) and Socket 754. One of the big cons with Sempron has for long been the lack of support for AMD64. The Socket 754-version is based on Athlon 64 and ahs the AMD64-architecture in the circuit, but AMD chose to disable the support for long. However recently AMD removed the limitation and new Sempron-processors has gained support for 64-bit.
The problem is that older Semprons are still available on the market and it might not be all that easy to find what stores carry the new models with AMD64 activated. At HardwareSecrets they’ve published a guide on how you identify a Sempron-processor with support for 64-bit.