For the "average" computer user who uses a computer for office tasks, emails, and internet browsing ... I doubt they would notice any difference between an A64 and a Sempy. The main differences between the two processor lines is the extra L-2 cache and instruction sets that the A64 has. People who do a lot of cpu intensive activities like audio/video encoding would notice the difference immediately.
In other words, a Sempron is fine for most people.