In case it was not clear before, the Sempron 2800+ is for the socket A like Bartons and the Sempron 3100+ is a 32 bit version of the Athlon 64 and runs on Socket 754 a completely different animal and much faster.
It sounds to me that you are going for a new system as you mentioned an A-64 2800+ so the 2800+ is just slightly faster then the 3100+ for a little more money but the difference may grow once the 64 bit capability of the 2800+ is utilized. The ratings are different such the 3100+ runs at a lower clock speed then the A-64 2800+ (I think) but it may overclock better. Either way unless you have a Socket A, STAY AWAY from the Sempron 2800+ or lower as they are just not worth it these days.