As you can see in my sign I have almost the same board as you. I have to agree with AMD808. Either you are lucky to find any CPu working on that board today or you just have to change almost everything to get running a Sempron skt A or a skt 754 (somewhere in this forum there are people claiming it Sempron 3100+ for skt 754 to be faster then the fastest of skt A Athlon XP 3200+).
The most common RAM are DDRs instead of PC133 today and mobo prices are cheap for skt A if you just want to get an skt A Sempron up and running.
Skt 754 are on its way out of market very soon. But it could be a cheap start to get access to Athlon 64 (64-bits CPU which can run 32-bit, that's today software). Sempron for this socket will just work in pure 32-bit and not for 64-bit (yet!!
So here are your choices! Keep what you have, take a chance on Sempron on your old mobo or just upgrade everything.
Abit AT8-32X (Xpress3200)/Athlon64 X2 4200+@stock/Corsair VS 2GB/Samsung SATA II 2x 250 GB/Club3D X1950XT GDDR3 256MB+ PowerColor Theatre 550 pro (PCIe x1)