Hmm, the Sempron is only just slightly slower than the Athlon, and is about £30 cheaper (and it's retail aswell).
http://www.hardcoreware.net/reviews/review-239-1.htm
Athlon XP 2500, and Sempron 2500 (with 256KB Cache) benchmark. The Sempron is only just slightly slower because of the smaller cache size, but the Sempron 3000+ Retail on Ebuyer has a 512KB L2 cache, so basically will be the same speed just £30 cheaper.