heh, buy a 2400, with the thorton core. I have oc'ed one to a stable 2.2ghz on stock cooling(runs hot though) but if you simply just go into your bios, you can easily have your sempron 2400 running @ about 2.0ghz simply by changing your FSB to 200. (do this only if you have PC-3200 ram) this is a very stable overclock, and can save you some dough if you dont want to buy a more expensive proc, but still get the same performance.
Mini guide to easily get your sempron 2400 running @ 2ghz
Go into Bios
Step Two: go to your FSB configuration and set it to 200
Step 3: save to bios and exit, your proc will now register in the bios as a AMD sempron 2000mhz.
Note: this only works if you have DDR-400 ram.