Just got your post in my other thread...
Basically the way I understand it is the Sempron is a cut-down Athlon XP. The later Athlon XPs with the Barton core had 512KB L2 cache and ran on a 333MHz and I think 400MHz FSB, whereas the earlier ones (Thoroughbred possibly - I'm not sure about the different cores) had 256KB and ran on a 266MHz FSB.
The Sempron Socket A runs on a 333MHz FSB with 256KB L2 cache, so as far as I can tell it's better than the first Athlon XPs. The 2400+ I think has the same (or similar) clock speed to the Athlon XP 2000+, but it runs on a faster FSB speed and is around the same price.
Also, to confuse the issue further, the top Sempron processor isn't Socket A; it's Socket 754 like some Athlon 64 processors. It's basically a cut-down Athlon 64 I think, stripped of its 64 bit capabilities. I see little point in this CPU - if you have a Socket 754 board you might as well get an Athlon 64 anyway! I suppose it offers a decent upgrade path if AMD keep making CPUs for Socket 754.
Hope that helps, even if not entirely accurate!