There's `possible' and there's `budget, with room to change if needed.' I'll go with the later. You can browse newegg yourself for the cheapest possible. Cheap doesn't always mean worth it, by the way.
Anyhow, budget recommendation:MotherboardCPU with Fan/Heatsink
Like, $115 with free shipping. Neither are garbage either. Good solid X2 5000+ cpu on a capable board with good reviews, room to expand and change things up if you need to in the future.