Uh very poorly, you will notice the Sempron is much cheaper than the Pentium4, or even the AthlonXP. Thats because its a cut down, low market version of the AthlonXP. Its the Celeron of AMD.
A 2600+ AthlonXP would perform much alike, about the same performance of a 2.6GHz Pentium4.
A 2600+ Sempron would probably (my best guess) perform about as good/fast as a 1.8GHz or a 2GHz Pentium4.
If your only doing Internet, word processing,etc, then it'd be fine, but if you want to play games and have a half fast computer, stay away from the Celeron and Sempron. Ever heard of the Duron? Sempron replaced it.
The cpu doesnt have as much cache as the AthlonXP does, which really cuts away from its performance, and a few other things here and there that cut it down, which is also why its soo much cheaper.