Bit of both. Well actually, it is both. Sempron is taking over from Athlon XP, so AMD are taking all the XP's off the market. But no fear. Sempron is just as good as AthlonXP. Well, nearly.
They are nearly as fast, but compared to Celeron D series. If I was you, just get a Sempron, but for 'true' athlon XP speeds, just add 200mhz to the Sempron rating. ie. AthlonXP 2000, get Sempron 2200.
Sempron run at a lower clock speed, but have higher Front Side Bus rating, which actually, nearly make up for the shortcomings in true clock speed. Hope this helps.