Yes it will support a sempron with a BIOS update to 1010
http://www.asus.com/support/download..._B.zip~zaqwedc
The Sempron is based off of an AthlonXP but has 256k of L2 cache instead of 512k.
You would probably be better off buying a 2500+ AthlonXP and setting the FSB to 400 turning it into a 3200+. You will need to have PC3200/DDR400 RAM and decent cooling in order to do that though.
The motherboard is a good board, but like everything in the computer world is becoming to be considered outdated. Not that that is at all bad, because it will still make a very capable machine that can deliver more performance than most people need.