The XP 2800+ is definitely faster than the Sempron 3000 but it's also
significantly more expensive as AMD has hiked the price tags on Bartons
recently. Nevertheless, keep in mind that Semprons are "budget" CPU's
while Bartons are top-of-the-line processors for the socket A platform.
I have recently purchased the XP 3000+ and am very impressed by its
performance so I'd recommend any Barton over any Sempron regardless of
cost.
FYI, the Sempron 3000+ IS a barton core cpu, only clocked like a Barton
2700+ XP would be. And cost wise, the XP 2800+ is only about $10 more than
the Sempron 3000+. The price on socket A cpu's have continued going up
over the last few months as the A64 CPU prices have fallen. I think it's
about reached a point where upgrading a socket A board is not cost
effective with the possible exception of a cheap barton core XP-M.