On the whole, the differences between Socket A Sempron and Athlon XP with identical model numbers are not significant. However, their technical characteristics are not significant either. So if you choose between Sempron / Socket A and Athlon XP, you have to take into account prices in the first place.
The differences between identically-numbered Athlon XP and Athlon 64 / 754 are often immense. Athlon XP is inferior here. Sometimes "the old guys" profit by their clocks, but on the whole Athlon 64 (often with a lower model number) looks obviously more promising.
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