Each motherboard only supports certain processors, and while I'd need the make and model to tell you which processors are supported, I can tell you right now that what you propose won't work. The reason is that a motherboard supports either Intel or AMD processors, not both, so if there's a P3 on it now it's an Intel board, meaning AMD processors are out of the question unless you replace the motherboard.
If you want to keep that motherboard, post back with its make and model number...if you don't know it you can use the free program Everest to find out. That will tell us what processors you can upgrade to. With it being an older model, though, your options will be limited. If you want to replace the motherboard, give a price range and we'll see what we can find for you.
Hope this helps,
Can any one give user advice on which Sempron I should opt for mutlitasking Office use.
I have a P3 system, which I want to replace the CPU with an AMD and 512MB RAM; I am price consicious, so decided for Sempron.
Which Sempron/Athlon is recommendable and works better, with Less Price possible??