Hey Jes,
I'll answer anything for a man looking out for his kid. Duron is an excellent chip. However, due to lack of L2 cache or shall I say smaller cache, and lower clock speed, its performance will not be as good as the Sempron 2500+ combo. Besides, the Sempron is based on the infamous Thoroughbred B core (well, infamous about 2~3 years ago). It is still an excellent core. I have Athlon XP2100+ which has the same core (and has the comparable performance with this Sempron), I can overclock it to 2.4Gig stable at 1.825 VCore, and it runs so cool. I don't know the overclockiing potential to this Sempron, but its performance, stock, is not too far behind the XP2500+ Barton in all aspects (I read a recent review).
Anyway, I know that the Duron combo is around $50, but spend the extra $25 and get the Sempron2500+. I think it will be worth every penny. Besides, once you get a computer cpu, its not something you'll want to swap out in a month or so (unless you have the money and desire for the latest bleeding edge technology, in which case you wouldn't be buying this in the first place). You'll have it for a while.
I hope everything works out well for you. Happy New Year everyone.