Wondering which one might be better, AMD 2800+ Barton core at 333 FSB Socket A or the new Sempron 3100+ Socket 754 chip. With the cutback on the lvl 1 cache and other changes with the Semprons, I don't know how big of difference the 754 socket will be compared to the Socket A board (although there's a little bit more of upgrade options later on down the road).

I have to replace the motherboard that the 2800+ is on for a friend and because of cost, I'm wondering which is better way to go, got about 150$ that we have to play with right now. The 939 socket is just way out of price range with having to get a cpu for that right now.  Just kind of wondering what you guys might think. Just replace the board, or get a new Sempron/board combo for about 130$.

The semprons cache might be less then the bartons but its cache is faster. Sempron is usually seen as a weak processor. In my country a second hand Barton 2500+ is more expensive then a new Sempron 2800+. I think the reason is that having an Athlon sounds much better than having a Sempron. Otherwise Sempron has a BUS of 1600 and 64bit. So excepting the name I would rather go for the Sempron.

No, the Sempron's rating is not comparable to the rating of the Athlon XP or Athlon 64.
A Sempron 3100+ (socket A) is approximately comparable to an Athlon XP 2600+ while a Sempron 3100+ at Sockel 754 is approximately the XP 2800+ with (IMHO) slight advantages for the XP.