Do you already have a CPU? If not I don't see how people can ignore the bang/buck of the Fry's ECS/AMD combos, which is $70 for a boxed with fan and 3 year warranty Sempron 2500+ and a ECS N2U400 motherboard that includes USB 2.0, 10/100 lan, and decent builtin sound.
My question to you would be first, what kind of motherboard will your existing case, power supply, and memory support?